Hydraulic driving system of construction machinery

ABSTRACT

A hydraulic drive system comprises directional flow control valves ( 10   a - f ) for selectively supplying a hydraulic fluid from a first hydraulic pump ( 1   a   , 1   b ), inflow control valves ( 201 - 203 ) disposed respectively in branch lines ( 150 A-C) branched from a supply line ( 100 ) for supplying a hydraulic fluid delivered from a second hydraulic pump ( 3   a   , 3   b ) to rod pushing-side chambers ( 5   a A,  5   b A,  6 A,  7 A) of hydraulic cylinders, a bypass flow control valve ( 204 ) disposed in a line ( 104 ) connecting the supply line ( 100 ) and a reservoir ( 2 ), and a controller ( 31 ) for computing control variables corresponding to operation command signals from control levers ( 32, 33 ) and controlling the inflow control valves ( 201 - 203 ) and the bypass flow control valve ( 204 ) in accordance with the computed control variables. Thus, the number of flow control valves and the length of piping required for their connection can be reduced, and a total pressure loss can be further reduced.

TECHNICAL FIELD

The present invention relates to a hydraulic drive system for aconstruction machine such as a hydraulic excavator, and moreparticularly to a hydraulic drive system for a construction machine,which is suitably used in the so-called super-large-sized hydraulicexcavator.

BACKGROUND ART

As disclosed in FIG. 9 of JP,A 9-328784, for example, there isconventionally known a hydraulic drive system for a constructionmachine, which is applied to a construction machine such as asuper-large-sized hydraulic excavator of a class having its own weightof 70 tons or more, in particular, the so-called backhoe type hydraulicexcavator including a swing body swingably mounted on a lower travelstructure and a multi-articulated front operating mechanism comprising aboom rotatably coupled to the swing body, an arm rotatably coupled tothe boom, and a bucket rotatably coupled to the arm to be open rearwardin a ground contact state.

Such a hydraulic drive system comprises two hydraulic pumps driven by afirst prime mover; two hydraulic pumps driven by a second prime mover; aboom hydraulic cylinder, an arm hydraulic cylinder and a buckethydraulic cylinder supplied with hydraulic fluids delivered from thefour hydraulic pumps for driving the boom, the arm and the bucket,respectively; a first group of directional flow control valves includinga boom directional flow control valve, an arm directional flow controlvalve and a bucket directional flow control valve for controllingrespective flows of the hydraulic fluids supplied from two of the fourhydraulic pumps to the boom hydraulic cylinder, the arm hydrauliccylinder and the bucket hydraulic cylinder; and a second group ofdirectional flow control valves including a boom directional flowcontrol valve, an arm directional flow control valve and a bucketdirectional flow control valve for controlling respective flows of thehydraulic fluids supplied from the other two of the four hydraulic pumpsto the boom hydraulic cylinder, the arm hydraulic cylinder and thebucket hydraulic cylinder. Then, by joining the hydraulic fluids fromboth the first group of directional flow control valves and the secondgroup of directional flow control valves together for each pair of theboom directional flow control valves, the arm directional flow controlvalves and the bucket directional flow control valve, and thereaftersupplying the joined hydraulic fluids respectively to the boom hydrauliccylinder, the arm hydraulic cylinder and the bucket hydraulic cylinder(i.e., by supplying hydraulic fluids usually used in two systemscovering from hydraulic excavator pumps to directional flow controlvalves in a joined manner), the hydraulic fluid can be supplied to eachhydraulic cylinder at a large flow rate required for the operation ofthe super-large-sized machine.

To supply the hydraulic fluid under a very high pressure at a very largeflow rate, main lines must be constructed of hoses, steel pipes or thelikes having very large diameters. However, because hoses practicallyavailable from the market at present have a maximum diameter of about 2inches, several (e.g., two or three) hoses must be laid side by side inpractice to meet the requirement. Accordingly, an allowable capacity asthe main lines is restricted as compared with the supply and drain flowrate required for a hydraulic actuator, and a relatively large pressureloss occurs in each of hoses constituting the main lines. Hence, a verylarge pressure loss is eventually generated in the whole of a hydrauliccircuit of the super-large-sized machine having long lines formed ofhoses, steel piles or the likes, flow control selector valves, etc. Thepressure loss increases an energy loss and causes another problem thatthe operating speed of the hydraulic actuator reduces and the workingefficiency deteriorates.

To cope with the problems mentioned above, as disclosed in FIGS. 1 and 2of the above-cited JP,A 9-328784, for example, a hydraulic drive systemfor a construction machine is also already proposed in which the numberof hoses and a total length of lines formed of steel pipes, etc. in asuper-large-sized machine are cut to reduce a total pressure loss.

That prior-art drive system comprises two hydraulic pumps driven by afirst prime mover; two hydraulic pumps driven by a second prime mover; aboom hydraulic cylinder, an arm hydraulic cylinder and a buckethydraulic cylinder supplied with hydraulic fluids delivered from thefour hydraulic pumps for driving the boom, the arm and the bucket,respectively; a boom directional flow control valve, an arm directionalflow control valve and a bucket directional flow control valve forcontrolling respective flows of the hydraulic fluids supplied from twoof the four hydraulic pumps to the boom hydraulic cylinder, the armhydraulic cylinder and the bucket hydraulic cylinder; a pair of boombottom-side inflow control valve and boom rod-side inflow control valve,a pair of arm bottom-side inflow control valve and arm rod-side inflowcontrol valve, and a pair of bucket bottom-side inflow control valve andbucket rod-side inflow control valve for controlling respective flows ofthe hydraulic fluids supplied from the other two of the four hydraulicpumps to rod pushing-side chambers and rod drawing-side chambers of theboom hydraulic cylinder, the arm hydraulic cylinder and the buckethydraulic cylinder without passing the boom directional flow controlvalve, the arm directional flow control valve and the bucket directionalflow control valve; and a pair of boom rod-side outflow control valveand boom bottom-side outflow control valve, a pair of arm rod-sideoutflow control valve and arm bottom-side outflow control valve, and apair of bucket rod-side outflow control valve and bucket bottom-sideoutflow control valve for controlling respective flows of the hydraulicfluids drained to a reservoir from the rod drawing-side chambers and therod pushing-side chambers of the boom hydraulic cylinder, the armhydraulic cylinder and the bucket hydraulic cylinder without passing theboom directional flow control valve, the arm directional flow controlvalve and the bucket directional flow control valve.

Then, for example, when performing boom-raising, arm-crowing andbucket-crowing operations, the hydraulic fluids are supplied from thefirst-mentioned two hydraulic pumps to the respective rod pushing-sidechambers of the boom hydraulic cylinder, the arm hydraulic cylinder andthe bucket hydraulic cylinder through the boom directional flow controlvalve, the arm directional flow control valve and the bucket directionalflow control valve, and the hydraulic fluids from the other twohydraulic pumps are joined with the flows of the hydraulic fluids, whichare supplied after having passed the respective directional flow controlvalves, through a separately provided common high-pressure line and thenthrough the boom bottom-side inflow control valve, the arm bottom-sideinflow control valve and the bucket bottom-side inflow control valve,which are disposed in respective lines branched from it, without passingthe boom directional flow control valve, the arm directional flowcontrol valve and the bucket directional flow control valve. The joinedhydraulic fluids are supplied to the respective rod pushing-sidechambers of the boom hydraulic cylinder, the arm hydraulic cylinder andthe bucket hydraulic cylinder.

Also, when performing boom-lowering, arm-dumping and bucket-dumpingoperations, the hydraulic fluids are supplied from the first-mentionedtwo hydraulic pumps to the respective rod drawing-side chambers of theboom hydraulic cylinder, the arm hydraulic cylinder and the buckethydraulic cylinder through the boom directional flow control valve, thearm directional flow control valve and the bucket directional flowcontrol valve, and the hydraulic fluids from the other two hydraulicpumps are joined from the common high-pressure line with the flows ofthe hydraulic fluids, which are supplied after having passed therespective directional flow control valves, through the boom rod-sideinflow control valve, the arm rod-side inflow control valve, and thebucket rod-side inflow control valve without passing the boomdirectional flow control valve, the arm directional flow control valve,and the bucket directional flow control valve. The joined hydraulicfluids are supplied to the respective rod drawing-side chambers of theboom hydraulic cylinder, the arm hydraulic cylinder and the buckethydraulic cylinder.

Thus, by providing not only ordinary hydraulic fluid supply routesextending from the first-mentioned hydraulic pumps through thedirectional flow control valves, but also hydraulic fluid supply routesextending from the other two hydraulic pumps through the commonhigh-pressure line without passing the directional flow control valves,the hydraulic fluid can be supplied to each hydraulic cylinder at alarge flow rate required for the operation of the super-large-sizedmachine. Further, the number of hoses and the total length of linesformed of steel pipes, etc. in the super-large-sized machine can be cutand the total pressure loss can be reduced.

DISCLOSURE OF THE INVENTION

However, the above-described prior art still has room for improvementsgiven below.

In general, a hydraulic cylinder has a large volume difference (e.g.,about 2:1) between a rod pushing-side chamber and a rod drawing-sidechamber thereof. Accordingly, when constructing an actualsuper-large-sized hydraulic excavator, components to be essentiallyadded for supply of the hydraulic fluid at the above-described largeflow rate are only six in total, i.e., the boom bottom-side inflowcontrol valve, the arm bottom-side inflow control valve and the bucketbottom-side inflow control valve for supplying the hydraulic fluid tothe respective pushing-side chambers, and the boom bottom-side outflowcontrol valve, the arm bottom-side outflow control valve and the bucketbottom-side outflow control valve for draining the return hydraulicfluid from the respective rod pushing-side chambers. The six flowcontrol valves connected to the respective rod drawing-side chambers arenot always required from the practical point of view. If those six flowcontrol valves connected to the respective rod drawing-side chambers canbe omitted, it should be possible to reduce the pressure loss caused bythose six directional flow control valves themselves. Also, it should bepossible to omit piping associated with those directional flow controlvalves and hence cut the pressure loss otherwise caused by such piping,and to realize a further reduction of the total pressure loss. Inaddition, a reduction in the number of hydraulic units, such as thedirectional flow control valves, could simplify layouts includingrouting of various pipes and arrangements of various units, particularlylayouts of hydraulic piping between the hydraulic pumps as hydraulicsources and actuators receiving the hydraulic fluids from the hydraulicsources.

In other words, such a point is not taken into account in theabove-described prior art and room for improvements still remains fromthat meaning.

An object of the present invention is to provide a hydraulic drivesystem for a construction machine, which can further reduce the numberof directional flow control valves and the length of piping forconnection, thereby realizing a further reduction of pressure loss as awhole, and which can simplify layouts of hydraulic piping betweenhydraulic sources and actuators receiving hydraulic fluids from thehydraulic sources with the reduced number of directional flow controlvalves.

To achieve the above object, the present invention provides a hydraulicdrive system for a construction machine, which drives and controls aplurality of hydraulic cylinders in the construction machine, thehydraulic drive system comprising a first hydraulic pump and a secondhydraulic pump driven by prime movers; directional flow control valvesfor selectively supplying a hydraulic fluid from the first hydraulicpump to rod pushing-side chambers and rod drawing-side chambers of theplurality of hydraulic cylinders; inflow control valves disposedrespectively in branch lines branched from one common line for supplyinga hydraulic fluid delivered from the second hydraulic pump to the rodpushing-side chambers of the hydraulic cylinders; a bypass flow controlvalve disposed in a line connecting the common line and a reservoir;input means for inputting operation command signals; and control meansfor computing control variables corresponding to the operation commandsignals from the input means and controlling the inflow control valvesand the bypass flow control valve in accordance with the computedcontrol variables.

In the present invention, when forming hydraulic fluid supply routes notpassing the directional flow control valves to supply the hydraulicfluid at a large flow rate to be adapted for a super-large-sizedmachine, the hydraulic fluid from the second hydraulic pump is suppliedfrom one common high-pressure line to the rod pushing-side chamber ofeach corresponding hydraulic cylinder via the respective branch lines.Supply flow rates at this time are controlled by the control meanscontrolling the inflow control valves disposed in the respective branchlines and the bypass flow control valve disposed in the line connectingthe common line and the reservoir in accordance with the controlvariables corresponding to the operation command signals from the inputmeans.

With those features, when supplying the hydraulic fluids to therespective rod pushing-side chambers of the hydraulic cylinders toperform, e.g., the boom-raising, arm-crowding and bucket-crowdingoperations, in addition to the supply of the hydraulic fluid from thefirst hydraulic pump through the corresponding directional flow controlvalves (directional flow control valves), the hydraulic fluid from thesecond hydraulic pump is joined with the hydraulic fluid, which issupplied through the directional flow control valves, through the inflowcontrol valves without passing the directional flow control valves. Thejoined hydraulic fluids are then supplied to the respective rodpushing-side chambers of the hydraulic cylinders. The return hydraulicfluids in this case are drained to the reservoir only via routes throughthe directional flow control valves. On the other hand, when supplyingthe hydraulic fluids to the respective rod drawing-side chambers of thehydraulic cylinders to perform, e.g., the boom-lowering, arm-dumping andbucket-dumping operations, the hydraulic fluid is supplied from thefirst hydraulic pump to the respective rod drawing-side chambers of thehydraulic cylinders through the directional flow control valves.

Thus, in consideration of the volume difference between the rodpushing-side chamber and the rod drawing-side chamber of each hydrauliccylinder, only the inflow control valves on the bottom side areadditionally provided to achieve the supply of the hydraulic fluid at alarge flow rate, while rod-side inflow control valves are omitted,whereby the pressure loss caused by the flow control valves can bereduced correspondingly. Also, since piping required for installation ofthe flow control valves is omitted and hence an accompanying pressureloss is eliminated, a total pressure loss can be further reduced. Inaddition, with a reduction in the number of the flow control valves, itis possible to simplify layouts including routing of various pipes andarrangements of various units, particularly layouts of hydraulic pipingbetween the hydraulic pumps as hydraulic sources and the actuators.

Also, to achieve the above object, the present invention provides ahydraulic drive system for a construction machine, which drives andcontrols a plurality of hydraulic cylinders in the construction machine,the hydraulic drive system comprising a first hydraulic pump and asecond hydraulic pump driven by prime movers; directional flow controlvalves for selectively supplying a hydraulic fluid from the firsthydraulic pump to rod pushing-side chambers and rod drawing-sidechambers of the plurality of hydraulic cylinders; outflow control valvesdisposed respectively in return fluid joining lines connected to the rodpushing-side chambers of the hydraulic cylinders; input means forinputting operation command signals; and control means for computingcontrol variables corresponding to the operation command signals fromthe input means and controlling the outflow control valves in accordancewith the computed control variables.

In the present invention, when forming hydraulic fluid drain routes notpassing the directional flow control valves to drain the hydraulic fluidat a large flow rate to be adapted for a super-large-sized machine, thereturn fluid joining lines are connected to the respective rodpushing-side chambers of the hydraulic cylinders. Drain flow rates atthis time are controlled by the control means controlling the outflowcontrol valves disposed in the respective return fluid joining lines andthe bypass flow control valve disposed in the line connecting the commonline and the reservoir in accordance with the control variablescorresponding to the operation command signals from the input means.

With those features, when supplying the hydraulic fluids to therespective rod drawing-side chambers of the hydraulic cylinders toperform, e.g., the boom-lowering, arm-dumping and bucket-dumpingoperations, the hydraulic fluid is supplied from the first hydraulicpump to the respective rod drawing-side chambers of the hydrauliccylinders through the corresponding directional flow control valves(directional flow control valves). The return hydraulic fluids aredrained to the reservoir as not only flows drained to the reservoir fromthe respective rod pushing-side chambers of the hydraulic cylindersthrough the directional flow control valves, but also flows branchedfrom the above flows and drained to the reservoir through the outflowcontrol valves and the return fluid joining lines without passing thedirectional flow control valves. On the other hand, when supplying thehydraulic fluids to the respective rod pushing-side chambers of thehydraulic cylinders to perform, e.g., the boom-raising, arm-crowding andbucket-crowding operations, the return hydraulic fluids from therespective rod drawing-side chambers are drained to the reservoir onlyvia routes through the directional flow control valves.

Thus, in consideration of the volume difference between the rodpushing-side chamber and the rod drawing-side chamber of each hydrauliccylinder, only the outflow control valves on the bottom side areadditionally provided to achieve the draining of the hydraulic fluid ata large flow rate, while rod-side outflow control valves are omitted,whereby the pressure loss caused by the flow control valves can bereduced correspondingly. Also, since piping required for installation ofthe flow control valves is omitted and hence an accompanying pressureloss is eliminated, a total pressure loss can be further reduced. Inaddition, with a reduction in the number of the flow control valves, itis possible to simplify layouts including routing of various pipes andarrangements of various units, particularly layouts of hydraulic pipingbetween the hydraulic pumps as hydraulic sources and the actuators.

Further, to achieve the above object, the present invention provides ahydraulic drive system for a construction machine, which drives andcontrols a plurality of hydraulic cylinders in the construction machine,the hydraulic drive system comprising a first hydraulic pump and asecond hydraulic pump driven by prime movers; directional flow controlvalves for selectively supplying a hydraulic fluid from the firsthydraulic pump to rod pushing-side chambers and rod drawing-sidechambers of the plurality of hydraulic cylinders; inflow control valvesdisposed respectively in branch lines branched from one common line forsupplying a hydraulic fluid delivered from the second hydraulic pump tothe rod pushing-side chambers of the hydraulic cylinders; outflowcontrol valves disposed respectively in return fluid joining linesconnected respectively to the branch lines; a bypass flow control valvedisposed in a line connecting the common line and a reservoir; inputmeans for inputting operation command signals; and control means forcomputing control variables corresponding to the operation commandsignals from the input means and controlling the inflow control valves,the outflow control valves and the bypass flow control valve inaccordance with the computed control variables.

Still further, to achieve the above object, the present inventionprovides a hydraulic drive system for a construction machine comprisinga travel body, a swing body swingably mounted onto the travel body, anda multi-articulated front operating mechanism made up of a boomrotatably coupled to the swing body, an arm rotatably coupled to theboom, and a bucket rotatably coupled to the arm, wherein the hydraulicdrive system comprises a boom hydraulic cylinder, an arm hydrauliccylinder, and a bucket hydraulic cylinder for driving the boom, the arm,and the bucket, respectively; at least one hydraulic pump mounted on theswing body; a common high-pressure line having one side connected to thedelivery side of the at least one hydraulic pump and the other sideextended to the side of the front operating mechanism; a boom branchline branched from the common high-pressure line and connected on theside opposite to the branched side to a rod pushing-side chamber of theboom hydraulic cylinder; a boom inflow control valve disposed near abranch position at which the boom branch line is branched from thecommon high-pressure line, and controlling a flow of a hydraulic fluidsupplied from the common high-pressure line to the rod pushing-sidechamber of the boom hydraulic cylinder; an arm branch line branched fromthe common high-pressure line at a position downstream of the branchposition of the boom branch line and connected on the side opposite tothe branched side to a rod pushing-side chamber of the arm hydrauliccylinder; an arm inflow control valve disposed near a branch position atwhich the arm branch line is branched from the common high-pressureline, and controlling a flow of a hydraulic fluid supplied from thecommon high-pressure line to the rod pushing-side chamber of the armhydraulic cylinder; a bucket branch line branched from the commonhigh-pressure line at a position downstream of the branch position ofthe boom branch line and connected on the side opposite to the branchedside to a rod pushing-side chamber of the bucket hydraulic cylinder; anda bucket inflow control valve disposed near the branch position at whichthe bucket branch line is branched from the common high-pressure line,and controlling a flow of a hydraulic fluid supplied from the commonhigh-pressure line to the rod pushing-side chamber of the buckethydraulic cylinder.

In the present invention, when forming hydraulic fluid supply routes notpassing the directional flow control valves to supply the hydraulicfluid at a large flow rate to be adapted for a super-large-sizedmachine, the common high-pressure line connected to the delivery side ofat least one hydraulic pump and extended to the side of the frontoperating mechanism is branched corresponding to an actual arrangementof respective actuators. First, a boom branch line connected to thebottom side of the boom hydraulic cylinder is branched from the commonhigh-pressure line at a position near the boom hydraulic cylinder. Then,an arm branch line connected to the bottom side of the arm hydrauliccylinder is branched from the common high-pressure line at a positiondownstream of the branch position of the boom branch line. The remainingpart of the common high-pressure line is constituted as a bucket branchline connected to the bottom side of the bucket hydraulic cylinder.Furthermore, a boom inflow control valve, an arm inflow control valve,and a bucket inflow control valve are disposed respectively in the boombranch line, the arm branch line, and the bucket branch line to controlflows of the hydraulic fluid from the common high-pressure line to therespective hydraulic cylinders.

With those features, when supplying the hydraulic fluids to therespective rod pushing-side chambers of the hydraulic cylinders toperform the boom-raising, arm-crowding and bucket-crowding operations,in addition to the ordinary supply of the hydraulic fluid to therespective rod pushing-side chambers of the hydraulic cylinders throughthe corresponding directional flow control valves, the hydraulic fluidfrom at least one hydraulic pump is joined with the hydraulic fluid,which is supplied through the directional flow control valves, throughthe inflow control valves without passing the directional flow controlvalves. The joined hydraulic fluids are then supplied to the respectiverod pushing-side chambers of the hydraulic cylinders. The returnhydraulic fluids in this case are drained to the reservoir only viaroutes through the directional flow control valves. On the other hand,when supplying the hydraulic fluids to the respective rod drawing-sidechambers of the hydraulic cylinders to perform, e.g., the boom-lowering,arm-dumping and bucket-dumping operations, the hydraulic fluid issupplied from the hydraulic pump to the respective rod drawing-sidechambers of the hydraulic cylinders through the directional flow controlvalves.

Thus, in consideration of the volume difference between the rodpushing-side chamber and the rod drawing-side chamber of each hydrauliccylinder, only the inflow control valves on the bottom side areadditionally provided to achieve the supply of the hydraulic fluid at alarge flow rate, while rod-side inflow control valves are omitted,whereby the pressure loss caused by the flow control valves can bereduced correspondingly. Also, since piping required for installation ofthe flow control valves is omitted and hence an accompanying pressureloss is eliminated, a total pressure loss can be further reduced. Inaddition, with a reduction in the number of the flow control valves, itis possible to simplify layouts including routing of various pipes andarrangements of various units, particularly layouts of hydraulic pipingbetween the hydraulic pumps as hydraulic sources and the actuators.

In the above hydraulic drive system for the construction machine,preferably, the inflow control valves are all disposed together in onecontrol valve unit.

Also, in the above hydraulic drive system for the construction machine,preferably, the hydraulic drive system further comprises at least one ofthree sets comprising a boom return fluid joining line branched from theboom branch line at a position nearer-to the boom hydraulic cylinderthan the boom inflow control valve and connected on the side opposite tothe branched side to a hydraulic reservoir, and a boom outflow controlvalve disposed in the boom return fluid joining line near a branchposition at which the boom return fluid joining line is branched fromthe boom branch line and controlling a flow of a hydraulic fluid drainedfrom the boom hydraulic cylinder to the hydraulic reservoir; an armreturn fluid joining line branched from the arm branch line at aposition nearer to the arm hydraulic cylinder than the arm inflowcontrol valve and connected on the side opposite to the branched side tothe hydraulic reservoir, and an arm outflow control valve disposed inthe arm return fluid joining line near a branch position at which thearm return fluid joining line is branched from the arm branch line andcontrolling a flow of a hydraulic fluid drained from the arm hydrauliccylinder to the hydraulic reservoir; and a bucket return fluid joiningline branched from the bucket branch line at a position nearer to thebucket hydraulic cylinder than the bucket inflow control valve andconnected on the side opposite to the branched side to the hydraulicreservoir, and a bucket outflow control valve disposed in the bucketreturn fluid joining line near a branch position at which the bucketreturn fluid joining line is branched from the bucket branch line andcontrolling a flow of a hydraulic fluid drained from the buckethydraulic cylinder to the hydraulic reservoir.

With those features, when the hydraulic fluids are supplied to therespective rod drawing-side chambers of the hydraulic cylinders in theboom-lowering, arm-dumping and bucket-dumping operations, a part of thehydraulic fluids returned from the rod drawing-side chambers at largeflow rates can be drained to the hydraulic reservoir without passing thedirectional flow control valves, and hence the smooth operation of thefront operating mechanism can be ensured.

In the above hydraulic drive system for the construction machine, morepreferably, the inflow control valves and the outflow control valves areall disposed together in one control valve unit.

Further, to achieve the above object, the present invention provides ahydraulic drive system comprising a first hydraulic pump and a secondhydraulic pump driven by prime movers; a plurality of hydrauliccylinders driven by hydraulic fluids delivered from the first and secondhydraulic pumps; a plurality of directional flow control valves forcontrolling respective flows of the hydraulic fluid supplied from thefirst hydraulic pump to the plurality of hydraulic cylinders; at leastone inflow control valve for controlling a flow of the hydraulic fluiddelivered from the second hydraulic pump and supplied to at least onerod pushing-side chamber among the plurality of hydraulic cylinderswithout passing the directional flow control valves; a bypass flowcontrol valve for returning the hydraulic fluid delivered from thesecond hydraulic pump to a reservoir; and a recovery flow control valvefor introducing the hydraulic fluid in at least one rod pushing-sidechamber among the plurality of hydraulic cylinders to a rod drawing-sidechamber thereof.

When supplying the hydraulic fluids to the respective rod pushing-sidechambers of the hydraulic cylinders to perform, e.g., the boom-raising,arm-crowding (arm-pushing) and bucket-crowding operations, the hydraulicfluid is supplied from the first hydraulic pump to the respective rodpushing-side chambers of the hydraulic cylinders through thecorresponding directional flow control valves (directional flow controlvalves), and the hydraulic fluid from the second hydraulic pump isadditionally joined with the above hydraulic fluid, which is suppliedthrough the directional flow control valves, through the inflow controlvalves without passing the directional flow control valves. The joinedhydraulic fluids are then supplied to the respective rod pushing-sidechambers of the hydraulic cylinders. The return hydraulic fluids in thiscase are drained to the reservoir only via routes through thedirectional flow control valves.

On the other hand, when supplying the hydraulic fluids to the respectiverod drawing-side chambers of the hydraulic cylinders to perform, e.g.,the boom-lowering, arm-dumping (arm-drawing) and bucket-dumpingoperations, the hydraulic fluid is supplied from the first hydraulicpump to the respective rod drawing-side chambers of the hydrauliccylinders through the directional flow control valves.

Thus, in consideration of the volume difference between the rodpushing-side chamber and the rod drawing-side chamber of each hydrauliccylinder, only the inflow control valves associated with the rodpushing-side chambers are additionally provided to achieve the supply ofthe hydraulic fluid at a large flow rate, while inflow control valvesassociated with the rod drawing-side chambers are omitted, whereby thepressure loss caused by the flow control valves can be reducedcorrespondingly. Also, since piping required for installation of theflow control valves is omitted and hence an accompanying pressure lossis eliminated, a total pressure loss can be further reduced. Inaddition, with a reduction in the number of the flow control valves, itis possible to simplify layouts including routing of various pipes andarrangements of various units, particularly layouts of hydraulic pipingbetween the hydraulic pumps as hydraulic sources and the actuators.

Further, because of the recovery flow control valve being provided inassociation with at least one hydraulic cylinder, when the hydraulicfluids are supplied to the respective rod drawing-side chambers of thehydraulic cylinders to perform, e.g., the boom-lowering, arm-dumping andbucket-dumping operations, the hydraulic fluid returned from the rodpushing-side chamber of the corresponding hydraulic cylinder is partlydrained to the reservoir via a route through the correspondingdirectional flow control. In parallel, the remaining return hydraulicfluid is introduced to the corresponding rod drawing-side chamberthrough the recovery flow control valve and is effectively utilized, asthe so-called recovery flow, for the operation of contracting thehydraulic cylinder. Regarding at least one hydraulic cylinder,therefore, the return hydraulic fluid from the rod pushing-side chambercan be effectively utilized as the recovery flow, which enables omissionof an outflow control valve having a large capacity associated with therod pushing-side chamber and an associated outflow line adapted for alarge flow rate. As a result, it is possible to further reduce thepressure loss for a reduction of the total pressure loss, and to furtherreduce the number of the flow control valves for more simplification ofthe layouts of hydraulic piping.

Still further, to achieve the above object, the present inventionprovides a hydraulic drive system for a construction machine comprisinga travel body, a swing body swingably mounted onto the travel body, anda multi-articulated front operating mechanism coupled to the swing bodyin a vertically angularly movable manner and made up of a boom, an armand a bucket, wherein the hydraulic drive system comprises a firsthydraulic pump and a second hydraulic pump driven by prime movers; aplurality of hydraulic cylinders including a boom hydraulic cylinder, anarm hydraulic cylinder and a bucket hydraulic cylinder supplied withhydraulic fluids delivered from the first and second hydraulic pumps todrive the boom, the arm, and the bucket, respectively; a plurality ofdirectional flow control valves for controlling respective flows of thehydraulic fluid supplied from the first hydraulic pump to the pluralityof hydraulic cylinders; at least one inflow control valve forcontrolling a flow of the hydraulic fluid delivered from the secondhydraulic pump and supplied to a rod pushing-side chamber of at leastthe boom hydraulic cylinder among the plurality of hydraulic cylinderswithout passing the directional flow control valves; a bypass flowcontrol valve for returning the hydraulic fluid delivered from thesecond hydraulic pump to a reservoir; and at least one recovery flowcontrol valve for introducing the hydraulic fluid in the rodpushing-side chamber of at least the boom hydraulic cylinder among theplurality of hydraulic cylinders to a rod drawing-side chamber thereof.

Still further, to achieve the above object, the present inventionprovides a hydraulic drive system for a construction machine comprisinga travel body, a swing body swingably mounted onto the travel body, anda multi-articulated front operating mechanism made up of a boomrotatably coupled to the swing body, an arm rotatably coupled to theboom, and a bucket rotatably coupled to the arm to be open forward in aground contact state, wherein the hydraulic drive system comprises atleast one first hydraulic pump and at least one second hydraulic pumpdriven by a plurality of prime movers; a plurality of hydrauliccylinders including a boom hydraulic cylinder, an arm hydraulic cylinderand a bucket hydraulic cylinder supplied with hydraulic fluids deliveredfrom the first and second hydraulic pump to drive the boom, the arm andthe bucket, respectively, and an opening/closing hydraulic cylindersupplied with the hydraulic fluids to open and close the bucket; aplurality of directional flow control valves for controlling respectiveflows of the hydraulic fluid supplied from the first hydraulic pump tothe plurality of hydraulic cylinders; at least two inflow control valvefor controlling respective flows of the hydraulic fluid delivered fromthe second hydraulic pump and supplied to rod pushing-side chambers ofat least the boom hydraulic cylinder and the bucket hydraulic cylinderamong the plurality of hydraulic cylinders without passing thedirectional flow control valves; a bypass flow control valve forreturning the hydraulic fluid delivered from the second hydraulic pumpto a reservoir; and at least two recovery flow control valve forintroducing the hydraulic fluids in the rod pushing-side chambers of atleast the boom hydraulic cylinder and the arm hydraulic cylinder amongthe plurality of hydraulic cylinders to rod drawing-side chambersthereof.

Still further, to achieve the above object, the present inventionprovides a hydraulic drive system for a construction machine comprisinga travel body, a swing body swingably mounted onto the travel body, anda multi-articulated front operating mechanism made up of a boomrotatably coupled to the swing body, an arm rotatably coupled to theboom, and a bucket rotatably coupled to the arm to be open rearward in aground contact state, wherein the hydraulic drive system comprises atleast one first hydraulic pump and at least one second hydraulic pumpdriven by a plurality of prime movers; a plurality of hydrauliccylinders including a boom hydraulic cylinder, an arm hydraulic cylinderand a bucket hydraulic cylinder supplied with hydraulic fluids deliveredfrom the first hydraulic pump and the second hydraulic pump to drive theboom, the arm and the bucket, respectively; a plurality of directionalflow control valves for controlling respective flows of the hydraulicfluid supplied from the first hydraulic pump to the plurality ofhydraulic cylinders; a plurality of inflow control valve for controllingrespective flows of the hydraulic fluid delivered from the secondhydraulic pump and supplied to rod pushing-side chambers of the boomhydraulic cylinders, the arm hydraulic cylinder and the bucket hydrauliccylinder without passing the directional flow control valves; a bypassflow control valve for returning the hydraulic fluid delivered from thesecond hydraulic pump to a reservoir; and at least one recovery flowcontrol valve for introducing the hydraulic fluid in the rodpushing-side chamber of at least the boom hydraulic cylinder among theplurality of hydraulic cylinders to a rod drawing-side chamber thereof.

Still further, to achieve the above object, the present inventionprovides a hydraulic drive system for a construction machine comprisinga travel body, a swing body swingably mounted onto the travel body, anda multi-articulated front operating mechanism made up of a boomrotatably coupled to the swing body, an arm rotatably coupled to theboom, and a bucket rotatably coupled to the arm to be open forward in aground contact state, wherein the hydraulic drive system comprises sixfirst hydraulic pumps and two second hydraulic pumps driven by aplurality of prime movers; a boom hydraulic cylinder, an arm hydrauliccylinder and a bucket hydraulic cylinder supplied with hydraulic fluidsdelivered from the first hydraulic pump and the second hydraulic pump todrive the boom, the arm and the bucket, respectively, and anopening/closing hydraulic cylinder supplied with the hydraulic fluids toopen and close the bucket; a plurality of boom directional flow controlvalves, a plurality of arm directional flow control valves, a pluralityof bucket directional flow control valves, and a plurality ofopening/closing directional flow control valves for controllingrespective flows of the hydraulic fluids supplied from the six firsthydraulic pumps to the boom hydraulic cylinder, the arm hydrauliccylinder, the bucket hydraulic cylinder, and the opening/closinghydraulic cylinder; a boom-raising inflow control valve, abucket-crowding inflow control valve and a bucket-dumping inflow controlvalve for controlling respective flows of the hydraulic fluids deliveredfrom the two second hydraulic pumps and supplied to a rod pushing-sidechamber of the boom hydraulic cylinder, a rod pushing-side chamber ofthe bucket hydraulic cylinder, and a rod drawing-side chamber of thebucket hydraulic cylinder without passing the plurality of boomdirectional flow control valves and the plurality of bucket directionalflow control valves; a bypass flow control valve for returning thehydraulic fluids delivered from the two second hydraulic pumps to areservoir; a boom recovery flow control valve and an arm recovery flowcontrol valve for introducing the hydraulic fluids in the respective rodpushing-side chambers of the boom hydraulic cylinder and the armhydraulic cylinder to rod drawing-side chambers thereof; and anopening/closing recovery flow control valve for introducing thehydraulic fluid in a rod drawing-side chamber of the opening/closinghydraulic cylinder to a rod pushing-side chamber thereof.

In the above hydraulic drive system for the construction machine,preferably, the inflow control valves are all disposed together in onecontrol valve unit.

In the above hydraulic drive system for the construction machine, morepreferably, the one control valve unit is disposed on the boom.

Also, in the above hydraulic drive system for the construction machine,preferably, check valves are disposed respectively in branch lines forsupplying the hydraulic fluid to the rod pushing-side chambers of thehydraulic cylinders.

Further, in the above hydraulic drive system for the constructionmachine, preferably, at least one of the inflow control valves, theoutflow control valves, and the bypass flow control valves isconstituted as a seat valve.

In the above hydraulic drive system for the construction machine, morepreferably, the seat valve is arranged such that an axis thereof liessubstantially in the horizontal direction.

With that feature, in operation, the front operating mechanism rotatesin the direction perpendicular to the axis of the seat valve. Therefore,the rotating operation of the front operating mechanism is avoided fromadversely affecting the opening/closing operation of the seat valve, andsmooth and reliable valve opening/closing operation can be ensured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to a first embodiment of thepresent invention along with a control system for it.

FIG. 2 is a side view showing the overall structure of a hydraulicexcavator driven by the hydraulic drive system shown in FIG. 1.

FIG. 3 is a functional block diagram showing, among detailed functionsof a controller shown in FIG. 1, control functions for inflow controlvalves, outflow control valves, and a bypass flow control valve.

FIG. 4 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to a second embodiment of thepresent invention along with a control system for it.

FIG. 5 is a side view showing the overall structure of a hydraulicexcavator driven by the hydraulic drive system shown in FIG. 4.

FIG. 6 is a functional block diagram showing, among detailed functionsof a controller shown in FIG. 4, control functions for inflow controlvalves, outflow control valves, and a bypass flow control valve.

FIG. 7 is a hydraulic circuit diagram showing the construction of ahydraulic drive system according to a third embodiment of the presentinvention.

FIG. 8 is a hydraulic circuit diagram showing the construction of ahydraulic drive system according to a fourth embodiment of the presentinvention.

FIG. 9 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to a fifth embodiment of thepresent invention along with a control system for it.

FIG. 10 is a functional block diagram showing, among detailed functionsof a controller shown in FIG. 9, control functions for inflow controlvalves, outflow control valves, a bypass flow control valve, and a boomrecovery flow control valve.

FIG. 11 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to a sixth embodiment of thepresent invention along with a control system for it.

FIG. 12 is a functional block diagram showing among detailed functionsof a controller shown in FIG. 11, control functions for inflow controlvalves, outflow control valves, a bypass flow control valve, and a boomrecovery flow control valve.

FIG. 13 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to a seventh embodiment of thepresent invention.

FIG. 14 shows extracted one of the flow control valves shown in FIG. 1.

FIG. 15 is an explanatory view showing the case in which the flowcontrol valve is constituted as a seat valve.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described below withreference to the drawings.

A first embodiment of the present invention will be described withreference to FIGS. 1 to 3. This embodiment represents the case in whichthe present invention is applied to the so-called super-large-sizedbackhoe type hydraulic excavator of a class having its own weight of 70tons, for example.

FIG. 1 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to this embodiment along with acontrol system for it. Referring to FIG. 1, the hydraulic drive systemof this embodiment comprises hydraulic pumps 1 a, 1 b driven by anengine (prime mover) 4 a, hydraulic pumps 3 a, 3 b driven by an engine 4b (allocation of the hydraulic pumps 1 a, 1 b, 3 a and 3 b with respectto the engines 4 a, 4 b is not limited to the above-described one, andmay be set as appropriate in consideration of horsepower distribution,etc.), boom hydraulic cylinders 5 a, 5 b, an arm hydraulic cylinder 6and a bucket hydraulic cylinder 7 which are supplied with hydraulicfluids delivered from the hydraulic pumps 1 a, 1 b, 3 a and 3 b, and ahydraulic reservoir 2.

The hydraulic pump 1 a is connected to the boom hydraulic cylinders 5 a,5 b, the arm hydraulic cylinder 6 and the bucket hydraulic cylinder 7through a first boom directional flow control valve (control valve) 10c, a first arm directional flow control valve 10 b, and a first bucketdirectional flow control valve 10 a, respectively. The hydraulic pump 1b is connected to the boom hydraulic cylinders 5 a, 5 b, the armhydraulic cylinder 6 and the bucket hydraulic cylinder 7 through asecond boom directional flow control valve 10 d, a second armdirectional flow control valve 10 e, and a second bucket directionalflow control valve 10 f, respectively. These directional flow controlvalves 10 a to 10 f constitute a directional flow control valve group10.

Rod pushing-side chambers (bottom-side hydraulic chambers) 5 aA, 5 bA ofthe boom hydraulic cylinders 5 a, 5 b are connected to the first andsecond boom directional flow control valves 10 c, 10 d via a main line105, and rod drawing-side chambers (rod-side hydraulic chambers) 5 aB, 5bB of the boom hydraulic cylinders 5 a, 5 b are connected to the firstand second boom directional flow control valves 10 c, 10 d via a mainline 115. Also, a rod pushing-side chamber 6A of the arm hydrauliccylinder 6 is connected to the first and second arm directional flowcontrol valves 10 b, 10 e via a main line 106, and a rod drawing-sidechamber 6B of the arm hydraulic cylinder 6 is connected to the first andsecond arm directional flow control valves 10 b, 10 e via a main line116. Further, a rod pushing-side chamber 7A of the bucket hydrauliccylinder 7 is connected to the first and second bucket directional flowcontrol valves 10 a, 10 f via a main line 107, and a rod drawing-sidechamber 7B of the bucket hydraulic cylinder 7 is connected to the firstand second bucket directional flow control valves 10 a, 10 f via a mainline 117.

On the other hand, the hydraulic pumps 3 a, 3 b are connected to themain lines 105, 106 and 107 via a delivery line 102 to which thehydraulic fluids delivered from the hydraulic pumps 3 a, 3 b areintroduced, then via a supply line 100 serving as a common high-pressureline which is connected at one side (left side as viewed in the drawing)thereof to the delivery line 102 and is extended to the side of a frontoperating mechanism 14 (described later), and then via branch lines150A, 150B and 150C branched from the other side of the supply line 100.

Of the branch lines 150A, 150B and 150C, the branch line 150A serving asa boom branch line is branched from the supply line 100 at a mostupstream position (among respective branched positions of the branchlines 150A, 150B and 150C). Also, the branch line 150B serving as an armbranch line is branched from the supply line 100 at a positiondownstream of the position at which the boom branch line 150A isbranched. Hence, the remaining branch line 150C serving as a bucketbranch line is also branched from the supply line 100 at a positiondownstream of the position at which the boom branch line 150A isbranched.

In the branch lines 150A, 150B and 150C, there are disposed respectivelya boom inflow control valve 201, an arm inflow control valve 202, and abucket inflow control valve 203 which are each constituted as, e.g., asolenoid proportional valve with a pressure compensating function andinclude respectively variable throttles 201A, 202A and 203A forcontrolling the flows of the hydraulic fluids supplied from thehydraulic pumps 3 a, 3 b to the rod pushing-side chambers 5 aA, 5 bA ofthe boom hydraulic cylinders, the rod pushing-side chamber 6A of the armhydraulic cylinder, and the rod pushing-side chamber 7A of the buckethydraulic cylinder to desired throttled flow rates. In this respect, theboom inflow control valve 201 is disposed near a branch position D1 atwhich the branch line 150A is branched from the supply line 100, and thearm inflow control valve 202 and the bucket inflow control valve 203 aredisposed near a branch position D2 at which the branch lines 150B, 150Care branched from the supply line 100.

Then, on the sides of the inflow control valve 201, 202 and 203 nearerto the hydraulic cylinders 5 a, 5 b, 6 and 7, check valves 151A, 151Band 151C are disposed respectively which allow the hydraulic fluids toflow from the hydraulic pumps 3 a, 3 b to the rod pushing-side chambers5 aA, 5 bA of the boom hydraulic cylinders, the rod pushing-side chamber6A of the arm hydraulic cylinder, and the rod pushing-side chamber 7A ofthe bucket hydraulic cylinder, but block off the hydraulic fluidsflowing in the reversed direction.

Further, the hydraulic reservoir 2 is connected to respective branchpositions in the branch lines 150A, 150B and 150C, which are locatednearer to the boom hydraulic cylinders 5 a, 5 b, the arm hydrauliccylinder 6, and the bucket hydraulic cylinder 7 than the inflow controlvalve 201, 202 and 203 and the check valves 151A, 151B and 151C, via areservoir line 103 for introducing the return hydraulic fluid to thehydraulic reservoir 2, then via a low-pressure drain line (return fluidjoining line) 101 connected at one side (left side as viewed in thedrawing) thereof to the reservoir line 103, and then via a branch line152A (boom return fluid joining line), a branch line 152B (arm returnfluid joining line), and a branch line 152C (bucket return fluid joiningline) which are connected to respective branch positions on the otherside of the drain line 101 (alternatively the hydraulic reservoir 2 maybe directly connected to the main lines 106, 107).

In the branch lines 152A, 152B and 152C, there are disposed respectivelya boom outflow control valve 211, an arm outflow control valve 212, anda bucket outflow control valve 213, which are each constituted as, e.g.,a solenoid proportional valve and include respectively variablethrottles 211A, 212A and 213A for controlling the flows of the hydraulicfluids drained to the hydraulic reservoir 2 from the rod pushing-sidechambers 5 aA, 5 bA of the boom hydraulic cylinders, the rodpushing-side chamber 6A of the arm hydraulic cylinder, and the rodpushing-side chamber 7A of the bucket hydraulic cylinder to desiredthrottled flow rates.

In this respect, the boom outflow control valve 211 is disposed near abranch position E1 at which the branch line 152A is branched from thedrain line 101 (also near a branch position F1 at which the branch line152A is connected to the branch line 150A). The arm outflow controlvalve 212 is disposed near a branch position E2 at which the branch line152B is branched from the drain line 101 (also near a branch position F2at which the branch line 152B is connected to the branch line 150B). Thebucket outflow control valve 213 is disposed near the branch position E2at which the branch line 152C is branched from the drain line 101 (alsonear a branch position F3 at which the branch line 152C is connected tothe branch line 150C).

The thus-arranged three inflow control valves 201, 202 and 203, threecheck valves 151A, 151B and 151C, and three outflow control valves 211,212 and 213 are disposed together in one control valve unit 190 (seeFIG. 2 described later) which is mounted to an upper surface (backsurface) of a boom 75.

Further, a line 104 is branched from the supply line 100 (or thedelivery line 102 as required). In this line 104, a bypass flow controlvalve 204 is disposed which is constituted as, e.g., a solenoidproportional valve with a pressure compensating function and suppliesthe hydraulic fluids delivered from the hydraulic pumps 3 a, 3 b to thesupply line 100 through a variable throttle 204A at a desired flow ratewhile returning the remaining hydraulic fluid to the hydraulic reservoir2 via the reservoir line 103. Additionally, between the delivery line102 and the reservoir line 103, a relief valve 205 is disposed tospecify a maximum pressure in the supply line 100 serving as ahigh-pressure line.

As shown in FIG. 2 described later, the hydraulic pumps 1 a, 1 b, 3 aand 3 b, the directional flow control valve group 10, the delivery line102, the reservoir line 103, the line 104, the bypass flow control valve21, the relief valve 22, etc. are disposed in a machine body 13. Thehydraulic cylinders 5 a, 5 b, 6 and 7, the supply line 100, the drainline 101, the branch lines 150A-C, 152A-C, the inflow control valves 201to 203, the check valves 151A-C, and the outflow control valves 211 to213 are disposed on the front operating mechanism 14 (see FIG. 2 aswell).

In the construction shown in FIG. 1, the lines 100, 102, 150A-C,105-107, 115-117, etc., serving as high-pressure lines, are each formedof, for example, a plurality of hoses (or steel pipes). The other lines101, 103, 152A-C, etc., serving as low-pressure lines, can be eachformed of a single large-diameter hose (or pipe) instead of a pluralityof hoses (or steel pipes).

FIG. 2 is a side view showing the overall structure of a hydraulicexcavator driven by the hydraulic drive system having theabove-described construction. In FIG. 2, the illustrated hydraulicexcavator is of the so-called backhoe excavator (backhoe type)comprising a travel device (travel body or lower travel structure) 79, amachine body (swing body or an upper swing structure) 13 swingablymounted onto the travel device 79 through a swing base bearing 78, and amulti-articulated front operating mechanism 14 (comprising a boom 75rotatably coupled to the machine body 13, an arm 76 rotatably coupled tothe boom 75, and a bucket 77 rotatably coupled to the arm 76 to be openrearward in a ground contact state), the front operating mechanism 14being vertically rotatably coupled to the machine body 13.

The boom hydraulic cylinders 5, the arm hydraulic cylinder 6 and thebucket hydraulic cylinder 7 are mounted, as shown, to the boom 75, thearm 76 and the bucket 77, respectively, to perform operations of boomraising (boom lowering), arm crowding (arm dumping) and bucket crowding(bucket dumping) with extension (contraction) thereof.

The swing body 13 is driven by a swing hydraulic motor (not shown)mounted therein to swing relative to the lower track structure (traveldevice) 79 through the swing base bearing 78. The travel device 79 isprovided with left and right travel hydraulic motors 79 b for drivingrespectively left and right crawler belts 79 a.

Returning to FIG. 1, a controller 31 is provided as a control unit forthe hydraulic drive system. The controller 31 receives operation signalsoutputted from control levers (input means) 32, 33 provided in a cab 13Aof the machine body 13, and outputs command signals to the directionalflow control valves 10 a-f, the inflow control valves 201 to 203, theoutflow control valves 211 to 213, and the bypass flow control valve204. The control levers 32, 33 are each movable in two orthogonaldirections. For example, the control lever 32 outputs a swing operationsignal and an arm operation signal when operated in the respectivedirections, and the control lever 33 outputs a boom operation signal anda bucket operation signal when operated in the respective directions.

FIG. 3 is a functional block diagram showing, among detailed functionsof the controller 31, control functions for the inflow control valves201 to 203, the outflow control valves 211 to 213, and the bypass flowcontrol valve 204, which constitute a principal part of this embodiment,other than general control functions of controlling the directional flowcontrol valves 10 a to 10 f in response to the operation signals fromthe control levers 32, 33. As shown in FIG. 3, the controller 31comprises a drive signal processing unit 231 for the boom inflow controlvalve 201, a drive signal processing unit 232 for the arm inflow controlvalve 202, a drive signal processing unit 233 for the bucket inflowcontrol valve 203, a drive signal processing unit 241 for the boomoutflow control valve 211, a drive signal processing unit 242 for thearm outflow control valve 212, a drive signal processing unit 243 forthe bucket outflow control valve 213, a drive signal processing unit 234for the bypass flow control valve 204, and a maximum value selector 235.

The drive signal processing units 231, 232, 233, 241, 242, 243 and 234receive corresponding operation input signals X from the control levers32, 33, and compute respective control signals S for the correspondingflow control valves 201, 202, 203, 211, 212, 213 and 204 (i.e., drivesignals applied to solenoid sectors 201B, 202B, 203B, 211B, 212B, 213Band 204B), followed by outputting the computed control signals to thecorresponding flow control valves. In this respect, each of the drivesignal processing units 231, 232, 233, 241, 242, 243 and 234 previouslystores, in the form of a table shown in FIG. 3, an operation patterndepending on the operation input signal X from the control lever (i.e.,a relationship between the operation input signal X from the controllever and a current value of a solenoid drive signal S for defining anopening area of each valve). In the operation table, a characteristic ofthe operation input signal X versus the solenoid drive signal S is setdepending on characteristics of each corresponding actuator so that anactuator operation characteristic optimum for an operator is obtainedwith respect to the operation input signal X.

More specifically, the boom-inflow drive signal processing unit 231receives a boom-raising operation input signal X from the control lever32, and computes a control signal S for the boom inflow control valve201 (i.e., a drive signal applied to the solenoid sector 201B) based onthe illustrated table, followed by outputting the computed controlsignal. The arm-inflow drive signal processing unit 232 receives anarm-crowding operation input signal X from the control lever 33, andcomputes a control signal S for the arm inflow control valve 202 (i.e.,a drive signal applied to the solenoid sector 202B) based on theillustrated table, followed by outputting the computed control signal.The bucket-inflow drive signal processing unit 233 receives abucket-crowding operation input signal X from the control lever 32, andcomputes a control signal S for the bucket inflow control valve 203(i.e., a drive signal applied to the solenoid sector 203B) based on theillustrated table, followed by outputting the computed control signal.

At this time, a maximum one of the boom-raising operation input signalX, the arm-crowding operation input signal X, and the bucket-crowdingoperation input signal X from the control levers 32, 33 is selected bythe maximum value selector 235 and then inputted to the bypass drivesignal processing unit 234. The bypass drive signal processing unit 234computes a control signal S for the bypass flow control valve 204 (i.e.,a drive signal applied to the solenoid sector 204B) based on theillustrated table, and outputs the computed control signal.

Further, the boom-outflow drive signal processing unit 241 receives aboom-lowering operation input signal X from the control lever 32, andcomputes a control signal S for the boom outflow control valve 211(i.e., a drive signal applied to the solenoid sector 211B) based on theillustrated table, followed by outputting the computed control signal.The arm-outflow drive signal processing unit 242 receives an arm-dumpingoperation input signal X from the control lever 33, and computes acontrol signal S for the arm outflow control valve 212 (i.e., a drivesignal applied to the solenoid sector 212B) based on the illustratedtable, followed by outputting the computed control signal. Thebucket-outflow drive signal processing unit 243 receives abucket-dumping operation input signal X from the control lever 32, andcomputes a control signal S for the bucket outflow control valve 213(i.e., a drive signal applied to the solenoid sector 213B) based on theillustrated table, followed by outputting the computed control signal.

The operation of this embodiment thus constructed will be describedbelow.

(1) Boom-Raising Operation

When the operator operates the control lever 32 in the directioncorresponding to the boom raising with intent to raise the boom for, byway of example, excavation, the produced operation input signal X isapplied as a boom raising command to the boom directional flow controlvalves 10 c, 10 d, thus causing their spools to shift in thecorresponding directions. As a result, the hydraulic fluids from thehydraulic pumps 1 a, 1 b are supplied to the rod pushing-side chambers 5aA, 5 bA of the boom hydraulic cylinders 5 a, 5 b via the main line 105.

On the other hand, the boom-inflow drive signal processing unit 231computes the drive signal S for the boom inflow control valve 201 inaccordance with the boom-raising operation input signal X from thecontrol lever 32 and outputs the computed drive signal S to the solenoidsector 201B of the boom inflow control valve 201. Simultaneously, inaccordance with the other operation signals (i.e., the boom-loweringoperation input signal, the arm-crowding and -dumping operation inputsignals, and the bucket-crowding and -dumping operation input signals),the corresponding drive signal processing units 232, 242, 233 and 243also compute the corresponding solenoid drive signals S. In this case,however, because the other operations are not commanded, each of thosedrive signal processing units computes a reference output (i.e., acurrent value, e.g., substantially zero, at which the valve will notopen) and outputs it. Then, the maximum value selector 235 selects amaximum one of the boom-raising operation input signal X, thearm-crowding operation input signal X, and the bucket-crowding operationinput signal X from the control levers 32, 33. However, because theother operations are not commanded, the bypass drive signal processingunit 234 eventually computes the drive signal S for the bypass flowcontrol valve 204 in accordance with the boom-raising operation inputsignal X from the control lever 32 and outputs the computed drive signalS to the solenoid sector 204B of the bypass flow control valve 204. As aresult, the bypass flow control valve 204 for returning the hydraulicfluids delivered from the hydraulic pumps 3 a, 3 b to the reservoir 2 isdriven to the closed side and the boom inflow control valve 201 isdriven to the open side, whereupon the hydraulic fluids delivered fromthe hydraulic pumps 3 a, 3 b are supplied to the rod pushing-sidechambers 5 aA, 5 bA of the boom hydraulic cylinders 5 a, 5 b via thedelivery line 102, the supply line 100, the branch line 150A, and theboom inflow control valve 201.

Accordingly, the hydraulic fluids delivered from the hydraulic pumps 3a, 3 b and supplied through the boom inflow control valve 201 are joinedwith the hydraulic fluids delivered from the hydraulic pumps 1 a, 1 band supplied through the boom directional flow control valves 10 c, 10d, thus causing the hydraulic fluids from the hydraulic pumps 1 a, 1 b,3 a and 3 b to flow into the rod pushing-side chambers 5 aA, 5 bA of theboom hydraulic cylinders 5 a, 5 b at a summed-up pump delivery rate.

On that occasion, the outflow rate of the return hydraulic fluids fromthe rod drawing-side chambers 5 aB, 5 bB of the boom hydraulic cylinders5 a, 5 b is about ½ of the inflow rate to the rod pushing-side chambers5 aA, 5 bA thereof because a volume ratio of the rod pushing-sidechamber to the rod drawing-side chamber of each cylinder is, forexample, about 2:1. In other words, the outflow rate of the returnhydraulic fluids is substantially equal to the inflow rate from the boomdirectional flow control valves 10 c, 10 d and can be accommodated bythose directional flow control valves 10 c, 10 d. Hence, the returnhydraulic fluids are returned to the reservoir 2 from the roddrawing-side chambers 5 aB, 5 bB via the main line 115 and meter-outthrottles (not shown) of the directional flow control valves 10 c, 10 d.

(2) Boom-Lowering Operation

When the operator operates the control lever 32 in the directioncorresponding to the boom lowering with intent to lower the boom for, byway of example, returning to the excavating position after loading theexcavated earth, the produced operation input signal X is applied as aboom lowering command to the boom directional flow control valves 10 c,10 d, thus causing their spools to shift in the correspondingdirections. As a result, the hydraulic fluids from the hydraulic pumps 1a, 1 b are supplied to the rod drawing-side chambers 5 aB, 5 bB of theboom hydraulic cylinders 5 a, 5 b via the main line 115.

At that time, because of the above-mentioned volume ratio of the rodpushing-side chamber to the rod drawing-side chamber, the outflow rateof the return hydraulic fluids from the rod pushing-side chambers 5 aA,5 bA is about twice the inflow rate to the rod drawing-side chambers 5aB, 5 bB. In this embodiment, therefore, the return hydraulic fluidscorresponding to a part (e.g., about ½) of that outflow rate arereturned to the reservoir 2 from the rod pushing-side chambers 5 aA, 5bA via the main line 105 and the meter-out throttles (not shown) of thedirectional flow control valves 10 c, 10 d. On the other hand, theboom-outflow drive signal processing unit 241 computes the drive signalS for the boom outflow control valve 211 in accordance with theboom-lowering operation input signal X from the control lever 32 andoutputs the computed drive signal S to the solenoid sector 211B of theboom outflow control valve 211. Simultaneously, the bypass drive signalprocessing unit 234 computes the drive signal S for the bypass flowcontrol valve 204 in accordance with the applied operation input signalX (X=0 in this case) and outputs the computed drive signal S to thesolenoid sector 204B of the bypass flow control valve 204. As a result,the bypass flow control valve 204 for returning the hydraulic fluidsdelivered from the hydraulic pumps 3 a, 3 b to the reservoir 2 is drivento the open side, and the boom outflow control valve 211 is driven tothe open side, whereupon the return hydraulic fluids from the rodpushing-side chambers 5 aA, 5 bA of the boom hydraulic cylinders 5 a, 5b are drained to the reservoir 2 via the branch line 150A, the branchline 152A, the boom outflow control valve 211, the drain line 101, andthe reservoir line 103.

(3) Arm-Crowding Operation

When the operator operates the control lever 33 in the directioncorresponding to the arm crowding with intent to crowd the arm for, byway of example, excavation, the produced operation input signal X isapplied as an arm crowding command to the arm directional flow controlvalves 10 b, 10 e, thus causing their spools to shift in thecorresponding directions. As a result, the hydraulic fluids from thehydraulic pumps 1 a, 1 b are supplied to the rod pushing-side chamber 6Aof the arm hydraulic cylinder 6 via the main line 106.

On the other hand, the arm-inflow drive signal processing unit 232computes the drive signal S for the arm inflow control valve 202 inaccordance with the arm-crowding operation input signal X from thecontrol lever 33 and outputs the computed drive signal S to the solenoidsector 202B of the arm inflow control valve 202. In the sole operationof arm crowding, the bypass drive signal processing unit 234 computesthe drive signal S for the bypass flow control valve 204 in accordancewith the arm-crowding operation input signal X from the control lever 33and outputs the computed drive signal S to the solenoid sector 204B ofthe bypass flow control valve 204. As a result, the bypass flow controlvalve 204 for returning the hydraulic fluids delivered from thehydraulic pumps 3 a, 3 b to the reservoir 2 is driven to the closed sideand the arm inflow control valve 202 is driven to the open side,whereupon the hydraulic fluids delivered from the hydraulic pumps 3 a, 3b are supplied to the rod pushing-side chamber 6A of the arm hydrauliccylinder 6 via the delivery line 102, the supply line 100, the branchline 150B, and the arm inflow control valve 202.

Accordingly, the hydraulic fluids delivered from the hydraulic pumps 3a, 3 b and supplied through the arm inflow control valve 202 are joinedwith the hydraulic fluids delivered from the hydraulic pumps 1 a, 1 band supplied through the arm directional flow control valves 10 b, 10 e,thus causing the hydraulic fluids from the hydraulic pumps 1 a, 1 b, 3 aand 3 b to flow into the rod pushing-side chamber 6A of the armhydraulic cylinder 6 at a summed-up pump delivery rate.

On that occasion, the outflow rate of the return hydraulic fluid fromthe rod drawing-side chamber 6B of the arm hydraulic cylinder 6 is, forexample, about ½ of the inflow rate to the rod pushing-side chamber 6A.In other words, the outflow rate of the return hydraulic fluid issubstantially equal to the inflow rate from the arm directional flowcontrol valves 10 b, 10 e and can be accommodated by those directionalflow control valves 10 b, 10 e. Hence, the return hydraulic fluids arereturned to the reservoir 2 from the rod drawing-side chamber 6B via themain line 116 and meter-out throttles (not shown) of the directionalflow control valves 10 b, 10 e.

(4) Arm-Dumping Operation

When the operator operates the control lever 33 in the directioncorresponding to the arm dumping with intent to dump the arm for, by wayof example, loading the excavated earth, the produced operation inputsignal X is applied as an arm dumping command to the arm directionalflow control valves 10 b, 10 e, thus causing their spools to shift inthe corresponding directions. As a result, the hydraulic fluids from thehydraulic pumps 1 a, 1 b are supplied to the rod drawing-side chamber 6Bof the arm hydraulic cylinder 6 via the main line 116.

At that time, because of the above-mentioned volume ratio of the rodpushing-side chamber to the rod drawing-side chamber, the outflow rateof the return hydraulic fluid from the rod pushing-side chamber 6A isabout twice the inflow rate to the rod drawing-side chamber 6B. In thisembodiment, therefore, the return hydraulic fluid corresponding to apart (e.g., about ½) of that outflow rate is returned to the reservoir 2from the rod pushing-side chamber 6B via the main line 106 and themeter-out throttles (not shown) of the directional flow control valves10 b, 10 e.

On the other hand, the arm-outflow drive signal processing unit 242computes the drive signal S for the arm outflow control valve 212 inaccordance with the arm-dumping operation input signal X from thecontrol lever 33 and outputs the computed drive signal S to the solenoidsector 212B of the arm outflow control valve 212. Simultaneously, thebypass drive signal processing unit 234 computes the drive signal S forthe bypass flow control valve 204 in accordance with the appliedoperation input signal X (X=0 in this case) and outputs the computeddrive signal S to the solenoid sector 204B of the bypass flow controlvalve 204. As a result, the bypass flow control valve 204 for returningthe hydraulic fluids delivered from the hydraulic pumps 3 a, 3 b to thereservoir 2 is driven to the open side, and the arm outflow controlvalve 212 is driven to the open side, whereupon the return hydraulicfluid from the rod pushing-side chamber 6A of the arm hydraulic cylinder6 is drained to the reservoir via the branch line 150B, the branch line152B, the arm outflow control valve 212, the drain line 101, and thereservoir line 103.

Consequently, the return hydraulic fluid from the rod pushing-sidechamber 6A of the arm hydraulic cylinder 6 is drained to the reservoirin a way divided into the hydraulic fluid drained to the reservoirthrough the arm directional flow control valves 10 b, 10 e and thehydraulic fluid drained to the reservoir through the arm outflow controlvalve 212.

(5) Bucket-Crowding Operation

When the operator operates the control lever 32 in the directioncorresponding to the bucket crowding with intent to crowd the bucketfor, by way of example, excavation, the produced operation input signalX is applied as an bucket crowding command to the bucket directionalflow control valves 10 a, 10 f, thus causing their spools to shift inthe corresponding directions. As a result, the hydraulic fluids from thehydraulic pumps 1 a, 1 b are supplied to the rod pushing-side chamber 7Aof the bucket hydraulic cylinder 7 via the main line 107.

On the other hand, the bucket-inflow drive signal processing unit 233computes the drive signal S for the bucket inflow control valve 203 inaccordance with the bucket-crowding operation input signal X from thecontrol lever 32 and outputs the computed drive signal S to the solenoidsector 203B of the bucket inflow control valve 203. In the soleoperation of bucket crowding, the bypass drive signal processing unit234 computes the drive signal S for the bypass flow control valve 204 inaccordance with the bucket-crowding operation input signal X from thecontrol lever 33 and outputs the computed drive signal S to the solenoidsector 204B of the bypass flow control valve 204. As a result, thebypass flow control valve 204 for returning the hydraulic fluidsdelivered from the hydraulic pumps 3 a, 3 b to the reservoir 2 is drivento the closed side and the bucket inflow control valve 203 is driven tothe open side, whereupon the hydraulic fluids delivered from thehydraulic pumps 3 a, 3 b are supplied to the rod pushing-side chamber 7Aof the bucket hydraulic cylinder 7 via the delivery line 102, the supplyline 100, the branch line 150C, and the bucket inflow control valve 203.

Accordingly, the hydraulic fluids delivered from the hydraulic pumps 3a, 3 b and supplied through the bucket inflow control valve 203 arejoined with the hydraulic fluids delivered from the hydraulic pumps 1 a,1 b and supplied through the bucket directional flow control valves 10a, 10 f, thus causing the hydraulic fluids from the hydraulic pumps 1 a,1 b, 3 a and 3 b to flow into the rod pushing-side chamber 7A of thebucket hydraulic cylinder 7 at a summed-up pump delivery rate. As in thecase of above (3), the return hydraulic fluid from the rod drawing-sidechamber 7B of the bucket hydraulic cylinder 7 on that occasion isreturned to the reservoir 2 from the rod drawing-side chamber 7B via themain line 117 and meter-out throttles (not shown) of the directionalflow control valves 10 a, 10 f.

(6) Bucket-Dumping Operation

When the operator operates the control lever 32 in the directioncorresponding to the bucket dumping with intent to dump the bucket for,by way of example, releasing the excavated earth above a bed of a dumptrack, the produced operation input signal X is applied as a bucketdumping command to the bucket directional flow control valves 10 a, 10f, thus causing their spools to shift in the corresponding directions.As a result, the hydraulic fluids from the hydraulic pumps 1 a, 1 b aresupplied to the rod drawing-side chamber 7B of the bucket hydrauliccylinder 7 via the main line 117.

At that time, as in the case of above (4), a part of the returnhydraulic fluid from the rod pushing-side chamber 7A is returned to thereservoir 2 from the rod pushing-side chamber 7 via the main line 107and the meter-out throttles (not shown) of the directional flow controlvalves 10 a, 10 f. On the other hand, the bucket-outflow drive signalprocessing unit 243 computes the drive signal S for the bucket outflowcontrol valve 213 in accordance with the bucket-dumping operation inputsignal X from the control lever 32 and outputs the computed drive signalS to the solenoid sector 213B of the bucket outflow control valve 213.Simultaneously, the bypass drive signal processing unit 234 computes thedrive signal S for the bypass flow control valve 204 in accordance withthe applied operation input signal X (X=0 in this case) and outputs thecomputed drive signal S to the solenoid sector 204B of the bypass flowcontrol valve 204. As a result, the bypass flow control valve 204 forreturning the hydraulic fluids delivered from the hydraulic pumps 3 a, 3b to the reservoir 2 is driven to the open side, and the bucket outflowcontrol valve 213 is driven to the open side, whereupon the returnhydraulic fluid from the rod pushing-side chamber 7A of the buckethydraulic cylinder 7 is drained to the reservoir via the branch line150C, the branch line 152C, the bucket outflow control valve 213, thedrain line 101, and the reservoir line 103.

Consequently, the return hydraulic fluid from the rod pushing-sidechamber 7A of the bucket hydraulic cylinder 7 is drained to thereservoir in a way divided into the hydraulic fluid drained to thereservoir through the bucket directional flow control valves 10 a, 10 fand the hydraulic fluid drained to the reservoir through the bucketoutflow control valve 213.

It is needless to say that, while the above description is made of, byway of example, in connection with the sole operation of boom raising,boom lowering, arm crowding, arm dumping, bucket crowding, or bucketdumping, composite control is performed in a combination of theabove-described control processes when two or more of the boom, the armand the bucket are operated in a combined manner.

With this embodiment, as described above, when forming hydraulic fluidsupply routes not passing the directional flow control valves 10 a-f tosupply the hydraulic fluid at a large flow rate in a backhoe typehydraulic excavator of an super-large class, the branch line 150Aleading to the rod pushing-side chambers 5 aA, 5 bA of the boomhydraulic cylinders is first branched from the supply line 100, servingas the common high-pressure line which is connected the delivery sidesof the hydraulic pumps 3 a, 3 b and extended to the side of the frontoperating mechanism 14, at a position near the boom hydraulic cylinders5 a, 5 b. Then, the branch line 150B leading to the rod pushing-sidechamber 6A of the arm hydraulic cylinder is branched from the supplyline 100 at a position downstream of the position at which the branchline 150A is branched, and the remaining part of the supply line 10 isconstituted as the branch line 150C leading to the rod pushing-sidechamber 7A of the bucket hydraulic cylinder. Further, the boom inflowcontrol valve 201, the arm inflow control valve 202, and the bucketinflow control valve 203 are disposed respectively in the branch lines150A, 150B and 150C to control the flows of the hydraulic fluids fromthe supply line 100 to the hydraulic cylinders 5 to 7.

When supplying the hydraulic fluids to the respective rod pushing-sidechambers 5 aA, 5 bA, 6A and 7A of the hydraulic cylinders 5 to 7 toperform the boom-raising, arm-crowding and bucket-crowding operations,in addition to the ordinary supply of the hydraulic fluids to therespective rod pushing-side chambers 5 aA, 5 bA, 6A and 7A of thehydraulic cylinders 5 to 7 through the directional flow control valves10 a-f, the hydraulic fluids from the hydraulic pumps 3 a, 3 b arejoined with the hydraulic fluids, which are supplied through thedirectional flow control valves 10 a-f, through the inflow controlvalves 201 to 203 without passing the directional flow control valves 10a-f. The joined hydraulic fluids are then supplied to the respective rodpushing-side chambers 5 aA, 5 bA, 6A and 7A of the hydraulic cylinders 5to 7. The return hydraulic fluids in this case are drained to thereservoir only via routes through the directional flow control valves 10a-f. On the other hand, when supplying the hydraulic fluids to therespective rod drawing-side chambers of the hydraulic cylinders 5 to 7to perform, e.g., the boom-lowering, arm-dumping and bucket-dumpingoperations, the hydraulic fluids are supplied from the hydraulic pumps 1a, 1 b to the respective rod drawing-side chambers 5 aB, 5 bB, 6B and 7Bof the hydraulic cylinders 5 to 7 through the directional flow controlvalves 10 a-f.

Thus, in consideration of the volume differences between the rodpushing-side chambers 5 aA, 5 bA, 6A and 7A and the rod drawing-sidechambers 5 aB, 5 bB, 6B and 7B of the hydraulic cylinders 5 to 7, onlythe inflow control valves 201, 202 and 203 in the bottom-side branchlines 150A-C are additionally provided to achieve the supply of thehydraulic fluid at a large flow rate, while rod-side inflow controlvalves are omitted, whereby the pressure loss caused by the flow controlvalves can be reduced correspondingly. Also, since piping required forinstallation of the flow control valves is omitted and hence anaccompanying pressure loss is eliminated, the pressure loss of theoverall hydraulic drive system can be further reduced. In addition, witha reduction in the number of the flow control valves, it is possible tosimplify layouts including routing of various pipes and arrangements ofvarious units, particularly layouts of hydraulic piping between thehydraulic pumps 3 a, 3 b as hydraulic sources and the hydrauliccylinders 5 a, 5 b, 6 and 7.

In addition to the super-large-sized hydraulic excavator describedabove, hydraulic excavators are classified into, for example, asmall-sized excavator having its own weight of not more than about 15tons, a medium-sized excavator having its own weight of not more thanabout 20 tons, and a large-sized excavator having its own weight ofabout 25 to 40 tons. The small- and medium-sized excavators are employedin relatively wide range of applications including ordinary constructionwork sites, etc. in Japan, while large-sized and super-large-sizedhydraulic excavators are adapted for large-scale excavation work and arepractically employed in digging of minerals in foreign mines in manycases. When those large-sized and super-large-sized hydraulic excavatorsare delivered to foreign customers from manufacturers in Japan, they aretransported by ship. It is therefore usual that the hydraulic excavatorsare not transported in the form of complete machines, but they areshipped in the form divided per related module (unit) and are assembledinto the complete machines after landing in sites. In general, ahydraulic drive system for a hydraulic excavator is constructed byconnecting hydraulic pumps, a reservoir, directional flow controlvalves, etc. with metal-made hydraulic pipes and hoses made of flexiblematerials. Because of having flexibility, the hoses can be easilyconnected and fixed at their opposite ends to corresponding mouthpiecesof the components as connection targets through field fitting of theactual parts in assembly work after landing. On the other hand, thehydraulic pipes are welded to the components as connection targets toform integral structures. In trying to weld the hydraulic pipes duringthe assembly after landing, however, required work becomes verycomplicated and difficult to perform. For that reason, it is preferableto transport the hydraulic excavator in the form divided into blocksobtained after finishing welding as far as possible within an allowablerange prior to the shipment, and to minimize the welding work requiredin sites. When dividing the hydraulic excavator into blocks to that end,the size of one block must be minimized because there are prescribedtransport restrictions in shipping or truck transportation along publicroads from a manufacturer's factory to a port.

With this embodiment, since the rod-side inflow control valves areomitted as described above, the size of each flow control valve unit canbe reduced when the inflow control valves are prepared in the form ofblocks to reduce the amount of welding work to a minimum, which isrequired after shipment to foreign customers and landing. Accordingly,it is possible to easily clear the prescribed transport restrictions inshipping or truck transportation along public roads from themanufacturer's factory to the port, and hence to improvetransportability.

Further, in this embodiment, the branch lines 152A, 152B and 152C aredisposed which are branched from the branch lines 150A, 150B and 150Cconnected to the rod pushing-side chambers 5 aA, 5 bA of the boomhydraulic cylinders, the rod pushing-side chamber 6A of the armhydraulic cylinder, and the rod pushing-side chamber 7A of the buckethydraulic cylinder, respectively, and which are led to the drain line101. The outflow control valves 211, 212 and 213 are disposedrespectively in the branch lines 152A, 152B and 152C. With such anarrangement, when the boom-lowering, arm-dumping and bucket-dumpingoperations are performed with the supply of the hydraulic fluids to therod drawing-side chambers 5 aB, 5 bB, 6B and 7B of the hydrauliccylinders 5 a, 5 b, 6 and 7, parts of the hydraulic fluids to bereturned at large flow rates from the rod pushing-side chambers 5 aA, 5bA, 6A and 7A thereof are drained to the hydraulic reservoir 2 throughthe outflow control valves 211, 212 and 213 without passing thedirectional flow control valves 10 a, 10 b, 10 e and 10 f. Consequently,the smooth operation of the front operating mechanism 14 can be ensured.

A second embodiment of the present invention will be described withreference to FIGS. 4 to 6. This embodiment represents the case in whichthe present invention is applied to the so-called loader typesuper-large-sized hydraulic excavator unlike the above first embodiment.

FIG. 4 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to this embodiment along with acontrol system for it. Identical components to those in FIG. 1 aredenoted by the same symbols, and a description of those components isnot repeated here as appropriate. As shown in Fig. 4, the hydraulicdrive system of this embodiment further comprises, as another hydrauliccylinder, a bucket opening/closing hydraulic cylinder 8 supplied withthe hydraulic fluids from the hydraulic pumps 1 a, 1 b. Correspondingly,the hydraulic pump la is connected to the bucket opening/closinghydraulic cylinder 8 through a first bucket opening/closing directionalflow control valve 10 g, and the hydraulic pump 1 b is connected to thebucket opening/closing hydraulic cylinder 8 through a second bucketopening/closing directional flow control valve 10 h. These directionalflow control valves 10 g, 10 h constitute the directional flow controlvalve group 10 together with the above-mentioned directional flowcontrol valves 10 a to 10 f. Further, a rod pushing-side chamber 8A ofthe bucket opening/closing hydraulic cylinder 8 is connected to thefirst and second bucket opening/closing directional flow control valves10 g, 10 h via a main line 108, and a rod drawing-side chamber 8B of thebucket opening/closing hydraulic cylinder 8 is connected to the firstand second bucket opening/closing directional flow control valves 10 g,10 h via a main line 118.

FIG. 5 is a side view showing the overall structure of a hydraulicexcavator driven by the hydraulic drive system having the constructiondescribed above. Identical components to those in FIG. 2 are denoted bythe same symbols, and a description of those components is omitted hereas appropriate. As shown in FIG. 5, the hydraulic excavator of thisembodiment is of the so-called loader type in which a bucket 77 providedin the multi-articulated front operating mechanism 14 is mounted to beopen forward in a ground contact state, and the bucket opening/closinghydraulic cylinder 8 is mounted to the bucket 77 as shown. Then,operations of boom raising (or boom lowering), arm pushing (or armdrawing), bucket crowding (or bucket dumping), and bucket closing(bucket opening=opening of a bucket opening portion 77B relative to abucket base portion 77A) are performed with extension (or contraction)of the boom hydraulic cylinders 5 a, 5 b, the arm hydraulic cylinder 6,the bucket hydraulic cylinder 7, and the bucket opening/closinghydraulic cylinder 8, respectively.

Of the branch lines 150A to 150C, as in the above first embodiment, thebranch line 150A serving as a boom branch line is branched from thesupply line 100 at a most upstream position, and the other branch line150B serving as an arm branch line and branch line 150C serving as abucket branch line are branched from the supply line 100 at a positiondownstream of the position at which the boom branch line 150A isbranched.

Also, as in the first embodiment, the boom inflow control valve 201, thearm inflow control valve 202, and the bucket inflow control valve 203are disposed near the above-mentioned branch positions D1, D2. Further,the boom outflow control valve 211, the arm outflow control valve 212,and the bucket outflow control valve 213 are disposed respectively nearthe above-mentioned branch positions E1, F1, branch positions E2, F2,and branch positions E2, F3. The inflow control valves 201, 202 and 203,the check valves 151A, 151B and 151C, and the outflow control valves211, 212 and 213 are disposed together in one control valve unit 190which is mounted to an upper surface (back surface) of the boom 75.Then, the supply line 100, the drain line 101, the branch lines 150A-C,152A-C, the inflow control valves 201 to 203, the check valves 151A-C,and the outflow control valves 211 to 213 are disposed on the frontoperating mechanism 14.

Returning to FIG. 4, a controller 31′ provided as a control unit for theabove-described hydraulic drive system receives operation signalsoutputted from the control levers 32, 33 and an additionally providedcontrol lever 34, and outputs command signals to the directional flowcontrol valves 10 a-h, the inflow control valves 201, 202 and 203, theoutflow control valves 211, 212 and 213, and the bypass flow controlvalve 204. The control lever 34 is of the type outputting operationsignals for opening and closing the bucket when operated. The controllever 34 may be replaced with a pedal operable by the operator's foot.

FIG. 6 is a functional block diagram showing, among detailed functionsof the controller 31′, control functions for the inflow control valves201, 202 and 203, the outflow control valves 211, 212 and 213, and thebypass flow control valve 204, which constitute a principal part of thisembodiment, other than general control functions of controlling thedirectional flow control valves 10 a to 10 h in response to theoperation signals from the control levers 32, 33 and 34. As shown inFIG. 6, the controller 31′ comprises, similarly to the controller 31 inthe above first embodiment, a drive signal processing unit 231 for theboom inflow control valve 201, a drive signal processing unit 232 forthe arm inflow control valve 202, a drive signal processing unit 233 forthe bucket inflow control valve 203, a drive signal processing unit 241for the boom outflow control valve 211, a drive signal processing unit242 for the arm outflow control valve 212, a drive signal processingunit 243 for the bucket outflow control valve 213, a drive signalprocessing unit 234 for the bypass flow control valve 204, and a maximumvalue selector 235.

In this embodiment, the arm-inflow drive signal processing unit 232receives an arm-pushing operation input signal X from the control lever33, and computes a control signal S for the arm inflow control valve 202(i.e., a drive signal applied to the solenoid sector 202B) based on theillustrated table, followed by outputting the computed control signal.Then, the maximum value selector 235 selects a maximum one of theboom-raising operation input signal X, the arm-pushing operation inputsignal X, and the bucket-crowding operation input signal X from thecontrol levers 32, 33. The selected maximum operation signal is inputtedto the bypass drive signal processing unit 234. The bypass drive signalprocessing unit 234 computes the control signal S for the bypass flowcontrol valve 204 and outputs the computed control signal. Further, thearm-outflow drive signal processing unit 242 receives an arm-drawingoperation input signal X from the control lever 33, and computes acontrol signal S for the arm outflow control valve 212 (i.e., a drivesignal applied to the solenoid sector 212B) based on the illustratedtable, followed by outputting the computed control signal.

The operation of this embodiment thus constructed will be describedbelow.

(1) Boom-Raising Operation

(2) Boom-Lowering Operation

These operations (1) and (2) are the same as those in the above firstembodiment, and hence a description thereof is omitted here.

(3) Arm-Pushing Operation

When the operator operates the control lever 33 in the directioncorresponding to the arm pushing with intent to push the arm for, by wayof example, excavation, the produced operation input signal X is appliedas an arm pushing command to the arm directional flow control valves 10b, 10 e, thus causing their spools to shift in the correspondingdirections. As a result, the hydraulic fluids from the hydraulic pumps 1a, 1 b are supplied to the rod pushing-side chamber 6A of the armhydraulic cylinder 6 via the main line 106.

On the other hand, the arm-inflow drive signal processing unit 232computes the drive signal S for the arm inflow control valve 202 inaccordance with the arm-pushing operation input signal X from thecontrol lever 33 and outputs the computed drive signal S to the solenoidsector 202B of the arm inflow control valve 202. In the sole operationof arm pushing, the bypass drive signal processing unit 234 computes thedrive signal S for the bypass flow control valve 204 in accordance withthe arm-pushing operation input signal X from the control lever 33 andoutputs the computed drive signal S to the solenoid sector 204B of thebypass flow control valve 204. As a result, the bypass flow controlvalve 204 for returning the hydraulic fluids delivered from thehydraulic pumps 3 a, 3 b to the reservoir 2 is driven to the closed sideand the arm inflow control valve 202 is driven to the open side,whereupon the hydraulic fluids delivered from the hydraulic pumps 3 a, 3b are supplied to the rod pushing-side chamber 6A of the arm hydrauliccylinder 6 via the delivery line 102, the supply line 100, the branchline 150B, and the arm inflow control valve 202.

Accordingly, the hydraulic fluids delivered from the hydraulic pumps 3a, 3 b and supplied through the arm inflow control valve 202 are joinedwith the hydraulic fluids delivered from the hydraulic pumps 1 a, 1 band supplied through the arm directional flow control valves 10 b, 10 e,thus causing the hydraulic fluids from the hydraulic pumps 1 a, 1 b, 3 aand 3 b to flow into the rod pushing-side chamber 6A of the armhydraulic cylinder 6 at a summed-up pump delivery rate.

On that occasion, the outflow rate of the return hydraulic fluid fromthe rod drawing-side chamber 6B of the arm hydraulic cylinder 6 is, forexample, about ½ of the inflow rate to the rod pushing-side chamber 6A.In other words, the outflow rate of the return hydraulic fluid issubstantially equal to the inflow rate from the arm directional flowcontrol valves 10 b, 10 e and can be accommodated by those directionalflow control valves 10 b, 10 e. Hence, the return hydraulic fluids arereturned to the reservoir 2 from the rod drawing-side chamber 6B via themain line 116 and meter-out throttles (not shown) of the directionalflow control valves 10 b, 10 e.

(4) Arm-Drawing Operation

When the operator operates the control lever 33 in the directioncorresponding to the arm drawing with intent to draw the arm afterreleasing the excavated earth, for example, the produced operation inputsignal X is applied as an arm crowding command to the arm directionalflow control valves 10 b, 10 e, thus causing their spools to shift inthe corresponding directions. As a result, the hydraulic fluids from thehydraulic pumps 10 a, 10 b are supplied to the rod drawing-side chamber6B of the arm hydraulic cylinder 6 via the main line 116.

At that time, because of the above-mentioned volume ratio of the rodpushing-side chamber to the rod drawing-side chamber, the outflow rateof the return hydraulic fluid from the rod pushing-side chamber 6A isabout twice the inflow rate to the rod drawing-side chamber 6B. In thisembodiment, therefore, the return hydraulic fluid corresponding to apart (e.g., about ½) of that outflow rate is returned to the reservoir 2from the rod pushing-side chamber 6B via the main line 106 and themeter-out throttles (not shown) of the directional flow control valves10 b, 10 e.

On the other hand, the arm-outflow drive signal processing unit 242computes the drive signal S for the arm outflow control valve 212 inaccordance with the arm-drawing operation input signal X from thecontrol lever 33 and outputs the computed drive signal S to the solenoidsector 212B of the arm outflow control valve 212. Simultaneously, thebypass drive signal processing unit 234 computes the drive signal S forthe bypass flow control valve 204 in accordance with the appliedoperation input signal X (X=0 in this case) and outputs the computeddrive signal S to the solenoid sector 204B of the bypass flow controlvalve 204. As a result, the bypass flow control valve 204 for returningthe hydraulic fluids delivered from the hydraulic pumps 3 a, 3 b to thereservoir 2 is driven to the open side, and the arm outflow controlvalve 212 is driven to the open side, whereupon the return hydraulicfluid from the rod pushing-side chamber 6A of the arm hydraulic cylinder6 is drained to the reservoir via the branch line 150B, the branch line152B, the arm outflow control valve 212, the drain line 101, and thereservoir line 103.

Consequently, the return hydraulic fluid from the rod pushing-sidechamber 6A of the arm hydraulic cylinder 6 is drained to the reservoirin a way divided into the hydraulic fluid drained to the reservoirthrough the arm directional flow control valves 10 b, 10 e and thehydraulic fluid drained to the reservoir through the arm outflow controlvalve 212.

(5) Bucket-Crowding Operation

(6) Bucket-Dumping Operation

These operations (5) and (6) are the same as those in the above firstembodiment, and hence a description thereof is omitted here.

The loader type hydraulic excavator to which this embodiment is appliedoperates in a typical case as follows. From a condition where the frontoperating mechanism 14 is positioned close to the machine body 13 in afolded state, the boom-raising, arm-pushing and bucket-crowdingoperations are performed to scoop earth and sand in front of the frontoperating mechanism into the bucket 77. Then, the bucket 77 is elevatedto a high level immediately after the scooping, and the bucket openingportion 77B is opened relative to the bucket base portion 77A so thatthe earth and sand in the bucket 77 is released onto, e.g., alarge-sized dump truck. Thereafter, the front operating mechanism 14 isreturned to the initial folded state positioned close to the machinebody 13 through substantially simultaneous operations of not only bucketclosing and bucket dumping, but also boom lowering and arm drawing.

It is needless to say that, while the above operations (1) to (6) aredescribed, by way of example, in connection with the sole operation ofboom raising, boom lowering, arm pushing, arm drawing, bucket crowding,or bucket dumping, composite control is performed in a combination ofthe above-described control processes for the operations (1) to (6) whentwo or more of the boom, the arm and the bucket are operated in acombined manner, including the above-mentioned typical case.

With this embodiment, as with the first embodiment, the pressure losscaused by the flow control valves can be reduced. Also, since pipingrequired for installation of the flow control valves is omitted andhence an accompanying pressure loss is eliminated, the pressure loss ofthe overall hydraulic drive system can be further reduced. In addition,with a reduction in the number of the flow control valves, it ispossible to simplify layouts including routing of various pipes andarrangements of various units, particularly layouts of hydraulic pipingbetween the hydraulic pumps 3 a, 3 b as hydraulic sources and thehydraulic cylinders 5 a, 5 b, 6 and 7.

A third embodiment of the present invention will be described withreference to FIG. 7.

FIG. 7 is a hydraulic circuit diagram showing a principal part of theconstruction of a hydraulic drive system according to this embodiment.Identical components to those in the first and second embodiments aredenoted by the same symbols, and a description of those components isomitted here as appropriate.

In the first and second embodiments, taking into account that the rodpushing-side chambers 5 aA, 5 bA of the boom hydraulic cylinders, therod pushing-side chamber 6A of the arm hydraulic cylinder, and the rodpushing-side chamber 7A of the bucket hydraulic cylinder have relativelylarge volume ratios, the boom inflow control valve 201, the arm inflowcontrol valve 202, and the bucket inflow control valve 203 are providedto control the supply of the hydraulic fluids from the hydraulic pumps 3a, 3 b to the rod pushing-side chambers 5 aA, 5 bA, 6A and 7A, and theboom outflow control valve 211, the arm outflow control valve 212, andthe bucket outflow control valve 213 are provided to control thedraining of the hydraulic fluids from the rod pushing-side chambers 5aA, 5 bA, 6A and 7A. However, the present invention is not limited tosuch an arrangement. When consideration is just required to focus ononly the supply of the hydraulic fluids to the rod pushing-side chambers5 aA, 5 bA of the boom hydraulic cylinders, the rod pushing-side chamber6A of the arm hydraulic cylinder, and the rod pushing-side chamber 7A ofthe bucket hydraulic cylinder, the outflow control valves 211, 212 and213, etc. (including the lines 101, 152A, 152B, 152C, etc.) can beomitted, and it is just required to provide only the boom inflow controlvalve 201, the arm inflow control valve 202, and the bucket inflowcontrol valve 203 which are related to the supply of the hydraulicfluids.

This embodiment represents the case implementing the technical conceptmentioned above. In this embodiment, the boom inflow control valve 201is provided while attention is paid in particular to the supply of thehydraulic fluids to the rod pushing-side chambers 5 aA, 5 bA (the latterbeing not shown) of the boom hydraulic cylinders, for example, in thebackhoe type hydraulic excavator described in the first embodiment andthe loader type hydraulic excavator described in the second embodiment.The present invention is not limited to such an arrangement of the boominflow control valve 201. In the case of the embodiment using the loadertype hydraulic excavator, for example, the arm inflow control valve 202may be provided instead of the boom inflow control valve 201.

With this embodiment, the number of at least the flow control valves andthe associated piping can be reduced or omitted in comparison with thecase of providing the inflow control valves associated with the roddrawing-side chambers as well. In this meaning, this embodiment can alsoprovide the above-described advantages specific to the presentinvention, such as a reduction of the pressure loss and simplificationof layouts.

A fourth embodiment of the present invention will be described withreference to FIG. 8.

FIG. 8 is a hydraulic circuit diagram showing a principal part of theconstruction of a hydraulic drive system according to this embodiment.Identical components to those in the first to third embodiments aredenoted by the same symbols, and a description of those components isomitted here as appropriate.

In contrast with the above third embodiment, when only the draining ofthe hydraulic fluids from the rod pushing-side chambers 5 aA, 5 bA, 6Aand 7A is required to be taken into consideration, it is sufficient toprovide only the outflow control valves 211, 212 and 213 with omissionof the inflow control valves 201, 202 and 203, etc., the hydraulic pumps3 a, 3 b, the prime mover 4 b, the lines 102, 100 and 104, respectiveportions of the lines 150A, 150B and 150C in which the inflow controlvalves 201, 202 and 203 are disposed, the bypass flow control valve 204,the relief valve 205, etc., which are used in the first and secondembodiments.

This embodiment represents the case implementing the technical conceptmentioned above. In this embodiment, the boom outflow control valve 211is provided while attention is paid in particular to the draining of thehydraulic fluids from the rod pushing-side chambers 5 aA, 5 bA (thelatter being not shown) of the boom hydraulic cylinders, for example, inthe backhoe type hydraulic excavator described in the first embodimentand the loader type hydraulic excavator described in the secondembodiment. The present invention is not limited to such an arrangementof the boom outflow control valve 211. In the case of the embodimentusing the loader type hydraulic excavator, for example, the arm outflowcontrol valve 212 may be provided instead of the boom outflow controlvalve 211.

With this embodiment, the number of at least the flow control valves andthe associated piping can be reduced or omitted in comparison with thecase of providing the outflow control valves associated with the roddrawing-side chambers as well. In this meaning, this embodiment can alsoprovide the above-described advantages specific to the presentinvention, such as a reduction of the pressure loss and simplificationof layouts.

A fifth embodiment of the present invention will be described withreference to FIGS. 9 and 10. This embodiment represents the case inwhich a recovery flow control valve is provided in association with theboom hydraulic cylinder. Identical components to those in the firstembodiments are denoted by the same symbols, and a description of thosecomponents is omitted here as appropriate.

FIG. 9 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to this embodiment along with acontrol system for it.

In FIG. 9, the hydraulic drive system of this embodiment is applied tothe backhoe type hydraulic excavator, shown in FIG. 2, described abovein the first embodiment. The hydraulic drive system of this embodimentdiffers from the hydraulic drive system, shown in FIG. 1, describedabove in the first embodiment as follows. The connecting line 105connected to the rod pushing-side chambers 5 aA, 5 bA of the boomhydraulic cylinders 5 a, 5 b and the connecting line 115 connected tothe rod drawing-side chambers 5 aB, 5 bB thereof are connected to eachother via a recovery line 220. In the recovery line 220 (on the frontdevice 14 side, though not shown), a boom recovery flow control valve221 is disposed which is constituted as, e.g., a solenoid proportionalvalve and includes a variable throttle 221A for controlling the flows ofthe hydraulic fluids from the rod pushing-side chambers 5 aA, 5 bA ofthe boom hydraulic cylinders 5 a, 5 b to the rod drawing-side chambers 5aB, 5 bB thereof to a desired throttled flow rate. Further, on the sideof the boom recovery flow control valve 221 nearer to the roddrawing-side chambers 5 aB, 5 bB, a check valve 222 is disposed whichallows the hydraulic fluids to flow from the rod pushing-side chambers 5aA, 5 bA to the rod drawing-side chambers 5 aB, 5 bB, but blocks off thehydraulic fluids from flowing in the reversed direction. With such anarrangement, the hydraulic fluids in the rod pushing-side chambers 5 aA,5 bA of the boom hydraulic cylinders 5 a, 5 b are introduced to the roddrawing-side chambers 5 aB, 5 bB.

Corresponding to the above-described arrangement, the branch line 152A,which is branched from the branch line 150A associated with the boomhydraulic cylinders 5 a, 5 b and is connected to the drain line 101, andthe boom outflow control valve 211 are omitted.

A controller 31A similar to the controller 31 in the first embodiment isprovided as a control unit for the hydraulic drive system having theabove-described construction. The controller 31A receives operationsignals outputted from the control levers 32, 33 provided in the cab 13Aof the machine body 13, and outputs command signals to not only thedirectional flow control valves 10 a-f, the inflow control valves 201 to203, the outflow control valves 212, 213, and the bypass flow controlvalve 204, but also the boom recovery flow control valve 221 in thisembodiment.

FIG. 10 is a functional block diagram showing, among detailed functionsof the controller 31A, control functions for the inflow control valves201 to 203, the outflow control valves 212, 213, the bypass flow controlvalve 204, and the boom recovery flow control valve 221, whichconstitute a principal part of this embodiment, other than generalcontrol functions of controlling the directional flow control valves 10a to 10 f in response to the operation signals from the control levers32, 33. In FIG. 10, the controller 31A in this embodiment differs fromthe controller 31 in the first embodiment, described above in connectionwith FIG. 3, in that the boom-lowering operation signal X from thecontrol lever 32 is inputted to a boom-recovery drive signal processingunit 251. The boom-recovery drive signal processing unit 251 receivesthe boom-lowering operation input signal X from the control lever 32,and computes a control signal S for the boom recovery flow control valve221 (i.e., a drive signal applied to a solenoid sector 221B thereof)based on the illustrated table, followed by outputting the computedcontrol signal.

The operation of this embodiment thus constructed will be describedbelow, taking as an example the operation of boom lowering, which is themost prominent feature of this embodiment, along with the operation ofboom raising for the comparison purpose.

(1) Boom-Raising Operation

When the operator operates the control lever 32 in the directioncorresponding to the boom raising with intent to raise the boom for, byway of example, excavation, the produced operation input signal X isapplied as a boom raising command to the boom directional flow controlvalves 10 c, 10 d, thus causing their spools to shift in thecorresponding directions. As a result, the hydraulic fluids from thehydraulic pumps 1 a, 1 b are supplied to the rod pushing-side chambers 5aA, 5 bA of the boom hydraulic cylinders 5 a, 5 b via the-main line 105.

On the other hand, the boom-inflow drive signal processing unit 231computes the drive signal S for the boom inflow control valve 201 inaccordance with the boom-raising operation input signal X from thecontrol lever 32 and outputs the computed drive signal S to the solenoidsector 201B of the boom inflow control valve 201. Simultaneously, inaccordance with the other operation signals (i.e., the boom-loweringoperation input signal, the arm-crowding and—dumping operation inputsignals, and the bucket-crowding and —dumping operation input signals),the corresponding drive signal processing units 232, 242, 233 and 243also compute the corresponding solenoid drive signals S. In this case,however, because the other operations are not commanded, each of thosedrive signal processing units computes a reference output (i.e., acurrent value, e.g., substantially zero, at which the valve will notopen) and outputs it. Then, the maximum value selector 235 selects amaximum one of the boom-raising operation input signal X, thearm-crowding operation input signal X, and the bucket-crowding operationinput signal X from the control levers 32, 33. However, because theother operations are not commanded, the bypass drive signal processingunit 234 eventually computes the drive signal S for the bypass flowcontrol valve 204 in accordance with the boom-raising operation inputsignal X from the control lever 32 and outputs the computed drive signalS to the solenoid sector 204B of the bypass flow control valve 204. As aresult, the bypass flow control valve 204 for returning the hydraulicfluids delivered from the hydraulic pumps 3 a, 3 b to the reservoir 2 isdriven to the closed side and the boom inflow control valve 201 isdriven to the open side, whereupon the hydraulic fluids delivered fromthe hydraulic pumps 3 a, 3 b are supplied to the rod pushing-sidechambers 5 aA, 5 bA of the boom hydraulic cylinders 5 a, 5 b via thedelivery line 102, the supply line 100, the branch line 150A, and theboom inflow control valve 201.

Accordingly, the hydraulic fluids delivered from the hydraulic pumps 3a, 3 b and supplied through the boom inflow control valve 201 are joinedwith the hydraulic fluids delivered from the hydraulic pumps 1 a, 1 band supplied through the boom directional flow control valves 10 c, 10d, thus causing the hydraulic fluids from the hydraulic pumps 1 a, 1 b,3 a and 3 b to flow into the rod pushing-side chambers 5 aA, 5 bA of theboom hydraulic cylinders 5 a, 5 b at a summed-up pump delivery rate.

On that occasion, the outflow rate of the return hydraulic fluids fromthe rod drawing-side chambers 5 aB, 5 bB of the boom hydraulic cylinders5 a, 5 b is about ½ of the inflow rate to the rod pushing-side chambers5 aA, 5 bA thereof because a volume ratio of the rod pushing-sidechamber to the rod drawing-side chamber of each cylinder is, forexample, about 2:1. In other words, the outflow rate of the returnhydraulic fluids is substantially equal to the inflow rate from the boomdirectional flow control valves 10 c, 10 d and can be accommodated bythose directional flow control valves 10 c, 10 d. Hence, the returnhydraulic fluids are returned to the reservoir 2 from the roddrawing-side chambers 5 aB, 5 bB via the main line 115 and the meter-outthrottles (not shown) of the directional flow control valves 10 c, 10 d.

(2) Boom-Lowering Operation

When the operator operates the control lever 32 in the directioncorresponding to the boom lowering with intent to lower the boom, by wayof example, after loading the excavated earth, the produced operationinput signal X is applied as a boom lowering command to the boomdirectional flow control valves 10 c, 10 d, thus causing their spools toshift in the corresponding directions. As a result, the hydraulic fluidsfrom the hydraulic pumps 1 a, 1 b are supplied to the rod drawing-sidechambers 5 aB, 5 bB of the boom hydraulic cylinders 5 a, 5 b via themain line 115.

At that time, because of the above-mentioned volume ratio of the rodpushing-side chamber to the rod drawing-side chamber, the outflow rateof the return hydraulic fluids from the rod pushing-side chambers 5 aA,5 bA is about twice the inflow rate to the rod drawing-side chambers 5aB, 5 bB. In this embodiment, therefore, the return hydraulic fluidscorresponding to a part (e.g., about ½) of that outflow rate arereturned to the reservoir 2 from the rod pushing-side chambers 5 aA, 5bA via the main line 105 and the meter-out throttles (not shown) of thedirectional flow control valves 10 c, 10 d. Simultaneously, theboom-recovery drive signal processing unit 251 computes the drive signalS for the boom recovery flow control valve 221 in accordance with theboom-lowering operation signal X from the control lever 32 and outputsthe computed drive signal S to the solenoid sector 221B of the boomrecovery flow control valve 221. As a result, the boom recovery flowcontrol valve 221 is driven to the open side. On this occasion, becauseholding pressures are generated in the rod pushing-side chambers 5 aA, 5bA of the boom hydraulic cylinders 5 a, 5 b due to the dead load of theboom 75, the remaining part of the hydraulic fluids from the rodpushing-side chambers 5 aA, 5 bA is introduced (recovered) to the roddrawing-side chambers 5 aB, 5 bB through the check valve 222 and theboom recovery flow control valve 221 upon opening of the boom recoveryflow control valve 221.

With this embodiment thus constructed, as with the above firstembodiment, when forming hydraulic fluid supply routes not passing thedirectional flow control valves 10 a-f to supply the hydraulic fluid ata large flow rate in a backhoe type hydraulic excavator of ansuper-large class, the branch line 150A leading to the rod pushing-sidechambers 5 aA, 5 bA of the boom hydraulic cylinders is first branchedfrom the supply line 100 serving as the common high-pressure line whichis connected the delivery sides of the hydraulic pumps 3 a, 3 b andextended to the side of the front operating mechanism 14. Then, thebranch line 150B leading to the rod pushing-side chamber 6A of the armhydraulic cylinder is branched from the supply line 100 at a positiondownstream of the position at which the branch line 150A is branched,and the remaining part of the supply line 10 is constituted as thebranch line 150C leading to the rod pushing-side chamber 7A of thebucket hydraulic cylinder. Further, the boom inflow control valve 201,the arm inflow control valve 202, and the bucket inflow control valve203 are disposed respectively in the branch lines 150A, 150B and 150C tocontrol the flows of the hydraulic fluids from the supply line 100 tothe hydraulic cylinders 5 to 7.

When supplying the hydraulic fluids to the respective rod pushing-sidechambers 5 aA, 5 bA, 6A and 7A of the hydraulic cylinders 5 to 7 toperform the boom-raising, arm-crowding and bucket-crowding operations,in addition to the ordinary supply of the hydraulic fluids to therespective rod pushing-side chambers 5 aA, 5 bA, 6A and 7A of thehydraulic cylinders 5 to 7 through the directional flow control valves10 a-f, the hydraulic fluids from the hydraulic pumps 3 a, 3 b arejoined with the hydraulic fluids, which are supplied through thedirectional flow control valves 10 a-f, through the inflow controlvalves 201 to 203 without passing the directional flow control valves 10a-f. The joined hydraulic fluids are then supplied to the respective rodpushing-side chambers 5 aA, 5 bA, 6A and 7A of the hydraulic cylinders 5to 7. The return hydraulic fluids in this case are drained to thereservoir only via routes through the directional flow control valves 10a-f.

On the other hand, when supplying the hydraulic fluids to the respectiverod drawing-side chambers of the hydraulic cylinders 5 to 7 to perform,e.g., the boom-lowering, arm-dumping and bucket-dumping operations, thehydraulic fluids are supplied from the hydraulic pumps 1 a, 1 b to therespective rod drawing-side chambers 5 aB, 5 bB, 6B and 7B of thehydraulic cylinders 5 to 7 through the directional flow control valves10 a-f.

Thus, in consideration of the volume differences between the rodpushing-side chambers 5 aA, 5 bA, 6A and 7A and the rod drawing-sidechambers 5 aB, 5 bB, 6B and 7B of the hydraulic cylinders 5 to 7, onlythe inflow control valves 201, 202 and 203 in the bottom-side branchlines 150A-C are additionally provided to achieve the supply of thehydraulic fluid at a large flow rate, while rod-side inflow controlvalves are omitted, whereby the pressure loss caused by the flow controlvalves can be reduced correspondingly. Also, since piping required forinstallation of the flow control valves is omitted and hence anaccompanying pressure loss is eliminated, the pressure loss of theoverall hydraulic drive system can be further reduced. In addition, witha reduction in the number of the flow control valves, it is possible tosimplify layouts including routing of various pipes and arrangements ofvarious units, particularly layouts of hydraulic piping between thehydraulic pumps 3 a, 3 b as hydraulic sources and the hydrauliccylinders 5 a, 5 b, 6 and 7.

Especially in this embodiment, as described in above (2), a total flowrate of the return hydraulic fluids from the rod pushing-side chambers 5aA, 5 bA of the boom hydraulic cylinders 5 a, 5 b during theboom-lowering operation is accommodated as a flow rate ordinarilydrained to the reservoir 2 through the meter-out throttles of thedirectional flow control valves 10 c, 10 d and a flow rate recovered tothe rod drawing-side chambers 5 aB, 5 bB through the boom recovery flowcontrol valve 221. With such an arrangement, regarding the boomhydraulic cylinders 5 a, 5 b, a part of the return hydraulic fluids(extra flows to be drained) from the rod drawing-side chambers 5 aB, 5bB is effectively utilized as a recovery flow. It is therefore possibleto omit an outflow control valve having a large capacity and anassociated outflow line adapted for a large flow rate, which correspondto the arm outflow control valve 212, the branch line 152B, the bucketoutflow control valve 213, and the branch line 151C. As a result, thepressure loss is reduced correspondingly and hence the pressure loss ofthe overall hydraulic drive system can be further reduced. In addition,further omission of the boom outflow control valve enables the layoutsof the hydraulic piping to be further simplified.

While the above description is made of, by way of example, the case ofrecovering the return hydraulic fluids for the boom hydraulic cylinders5 a, 5 b from the rod pushing-side chambers 5 aA, 5 bA to the roddrawing-side chambers 5 aB, 5 bB, the present invention is not limitedto that arrangement. The return hydraulic fluid may be recovered fromthe rod drawing-side chamber to the rod pushing-side chamber in asimilar manner for the arm hydraulic cylinder 6 and the bucket hydrauliccylinder 7 with omission of the arm outflow control valve 212, thebranch line 152B, the bucket outflow control valve 213, and the branchline 152C. These modifications can also provide similar advantages tothose described above.

A sixth embodiment of the present invention will be described withreference to FIGS. 11 and 12. This embodiment represents the case inwhich the return hydraulic fluids are recovered in a loader typesuper-large-sized hydraulic excavator like the above fifth embodiment.

FIG. 11 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to this embodiment along with acontrol system for it. Identical components to those in the second andfifth embodiments are denoted by the same symbols, and a description ofthose components is omitted here as appropriate.

In FIG. 11, the hydraulic drive system of this embodiment is applied tothe loader type hydraulic excavator, shown in FIG. 5, described above inthe second embodiment. The hydraulic drive system of this embodimentdiffers from the hydraulic drive system, shown in FIG. 9, describedabove in the fifth embodiment as follows. First, as an additionalcylinder, a bucket opening/closing hydraulic cylinder 8 similar to thatused in the second embodiment is further provided which is supplied withthe hydraulic fluids from the hydraulic pumps 1 a, 1 b. Correspondingly,the hydraulic pump 1 a is connected to the bucket opening/closinghydraulic cylinder 8 through a first bucket opening/closing directionalflow control valve 10 g, and the hydraulic pump 1 b is connected to thebucket opening/closing hydraulic cylinder 8 through a second bucketopening/closing directional flow control valve 10 h. These directionalflow control valves 10 g, 10 h constitute the directional flow controlvalve group 10 together with the above-mentioned directional flowcontrol valves 10 a to 10 f. Further, a rod pushing-side chamber 8A ofthe bucket opening/closing hydraulic cylinder 8 is connected to thefirst and second bucket opening/closing directional flow control valves10 g, 10 h via a main line 108, and a rod drawing-side chamber 8B of thebucket opening/closing hydraulic cylinder 8 is connected to the firstand second bucket opening/closing directional flow control valves 10 g,10 h via a main line 118.

Further, among the branch lines 150A, 150B and 150C branched, in theabove fifth embodiment, from the other side of the supply line 100having one end (left side as viewed in the drawing) connected to thedelivery line 102 of the hydraulic pumps 3 a, 3 b, the branch line 150Band the arm inflow control valve 202 both associated with the armhydraulic cylinder 6 are omitted in this sixth embodiment. This omissionis based on the meaning given below. In the case of the loader typehydraulic excavator, unlike the backhoe type, ports of the arm hydrauliccylinder 6 are positioned closer to the machine body 13 than those inthe boom hydraulic cylinders 5 a, 5 b from its specific structure (seeFIG. 5). As a result, the lines 106, 116 extending from the ordinary armcontrol valves 10 b, 10 e to the arm hydraulic cylinder 6 can be setrelatively short and can be easily constructed. In some cases,therefore, the merit resulting from the provision of the arm inflowcontrol valve for supplying the hydraulic fluid at a large flow ratewithout passing the ordinary control valves is not so significant.

As another major feature of this embodiment, in addition to the recoveryline 220, the boom recovery flow control valve 221 and the check valve222 which are disposed for the boom hydraulic cylinders 5 a, 5 b in theabove fourth-embodiment, a similar arrangement is also provided for thearm hydraulic cylinder 6. More specifically, the connecting line 106connected to the rod pushing-side chamber 6A of the arm hydrauliccylinder 6 and the connecting line 116 connected to the rod drawing-sidechamber 6B thereof are connected to each other via a recovery line 223.In the recovery line 223, an arm recovery flow control valve 224 isdisposed which is constituted as, e.g., a solenoid proportional valveand includes a variable throttle 224A for controlling the flow of thehydraulic fluid from the rod pushing-side chamber 6A of the armhydraulic cylinder 6 to the rod drawing-side chamber 6B thereof to adesired throttled flow rate. Further, on the side of the arm recoveryflow control valve 224 nearer to the rod drawing-side chamber 6B, acheck valve 225 is disposed which allows the hydraulic fluid to flowfrom the rod pushing-side chamber 6A to the rod drawing-side chambers6B, but blocks off the hydraulic fluid from flowing in the reverseddirection. With such an arrangement, the hydraulic fluid in the rodpushing-side chamber 6A of the arm hydraulic cylinder 6 is introduced tothe rod drawing-side chamber 6B. It is hence possible to omit the branchline 152B and the outflow control valve 212 both associated with the armhydraulic cylinder 6, which are provided in the fifth embodiment shownin FIG. 9.

That omission is based on the meaning given below. In the case of theloader type hydraulic excavator, unlike the backhoe type, a holdingpressure is always generated in the rod pushing-side chamber 6A of thearm hydraulic cylinder due to the dead load of the arm 6 from itsspecific structure. Therefore, the arrangement of providing the armrecovery flow control valve 224 and introducing (recovering) thehydraulic fluid drained from the rod pushing-side chamber 6A to the roddrawing-side chamber 6B is simpler and more effective than providing theoutflow control valve.

In addition, based on the above-described features, no recovery flowcontrol valve is provided for the bucket 77 (because, in spite of theloader type, a holding pressure is not always generated in the rodpushing-side chamber 7A for the bucket 77 depending on the posture ofthe front operating mechanism 14 unlike the arm 75 and the arm 76) sothat the flow rate of the drained hydraulic fluid is all absorbed by thedirectional flow control valves 10 g, 10 h. Thus, the branch line 152Cand the outflow control valve 213 both associated with the buckethydraulic cylinder 7, which are provided in the fifth embodiment, areomitted. As a result, it is possible to omit the low-pressure drain line101 which is provided in the fifth embodiment and has one side (leftside as viewed in the drawing) connected to the reservoir line 103 forintroducing the return hydraulic fluid to the hydraulic reservoir 2.

Moreover, for the bucket hydraulic cylinder 7, a branch line 153C isadditionally branched from the other side of the supply line 100 (at aposition D3 where it is also branched from the line 150C). In thisbranch line 153C, a bucket inflow control valve 208 is disposed which isconstituted as, e.g., a solenoid proportional valve with a pressurecompensating function and includes a variable throttle 208A forcontrolling the flow of the hydraulic fluids from the hydraulic pumps 3a, 3 b to the rod drawing-side chamber 7B of the bucket hydrauliccylinder 7 to a desired flow rate. Further, on the side of the bucketinflow control valve 208 nearer to the bucket hydraulic cylinder 7, acheck valve 154C is disposed which allows the hydraulic fluid to flowfrom the hydraulic pumps 3 a, 3 b to the rod drawing-side chambers 7B ofthe bucket hydraulic cylinder, but blocks off the hydraulic fluid fromflowing in the reversed direction.

On the other hand, for the bucket opening/closing hydraulic cylinder 8,a circuit arrangement is added to provide a different recovery function(operating in the reversed direction) from those for the boom hydrauliccylinders 5 a, 5 b and the arm hydraulic cylinder 6. More specifically,the connecting line 108 connected to the rod pushing-side chamber 8A ofthe bucket opening/closing hydraulic cylinder 8 and the connecting line118 connected to the rod drawing-side chamber 8B thereof are connectedto each other via a recovery line 226. In the recovery line 226, abucket opening/closing recovery flow control valve 227 is disposed whichis constituted as, e.g., a solenoid proportional valve and includes avariable throttle 227A for controlling the flow of the hydraulic fluidfrom the rod drawing-side chamber 8B of the bucket opening/closinghydraulic cylinder 8 to the rod pushing-side chamber 8A thereof to adesired throttled flow rate. Further, on the side of the bucketopening/closing recovery flow control valve 227 nearer to the rodpushing-side chamber 8A, a check valve 228 is disposed which allows thehydraulic fluid to flow from the rod drawing-side chamber 8B to the rodpushing-side chambers 8A, but blocks off the hydraulic fluid fromflowing in the reversed direction. With such an arrangement, thehydraulic fluid in the rod drawing-side chamber 8B of the bucketopening/closing hydraulic cylinder 8 is introduced to the rodpushing-side chamber 8A.

The inflow control valves 201, 203 and 208 and the check valves 151A,151C and 154C are disposed together in one control valve unit 190′(though not shown, at the same position as the control valve unit 190 inFIG. 5) which is mounted to the upper surface (back surface) of the boom75. Then, the supply line 100, the branch lines 150A, 150C and 153C, theinflow control valves 201, 203 and 208, the check valves 151A, 151C and154C, the recovery flow control valves 221, 224 and 227, and the checkvalves 222, 225 and 228 are disposed on the front operating mechanism14.

A controller 31′A provided as a control unit for the hydraulic drivesystem having the above-described construction receives operationsignals outputted from the control levers 32, 33 and the control lever34 additionally provided as in the second embodiment, and outputscommand signals to the directional flow control valves 10 a-h, theinflow control valves 201, 203 and 208, the bypass flow control valve204, the boom recovery flow control valve 221, the arm recovery flowcontrol valve 224, and the bucket opening/closing recovery flow controlvalve 227.

FIG. 12 is a functional block diagram showing, among detailed functionsof the controller 31′A, control functions for the inflow control valves201, 203 and 208, the bypass flow control valve 204, the boom recoveryflow control valve 221, the arm recovery flow control valve 224, and thebucket opening/closing recovery flow control valve 227, which constitutea principal part of this embodiment, other than general controlfunctions of controlling the directional flow control valves 10 a to 10f in response to the operation signals from the control levers 32, 33and 34. As shown in FIG. 12, the controller 31′A does not include thedrive signal processing unit 232 for the arm inflow control valve 202,the drive signal processing unit 242 for the arm outflow control valve212, and the drive signal processing unit 243 for the bucket outflowcontrol valve 213, which are provided in the controller 31′ in the fifthembodiment. In contrast, a drive signal processing unit 253 for thebucket inflow control valve 208, a drive signal processing unit 252 forthe arm recovery flow control valve 224, and a drive signal processingunit 254 for the bucket opening/-closing recovery flow control valve 227are newly provided in the controller 31′A.

The bucket-inflow drive signal processing unit 253 receives abucket-dumping operation input signal X from the control lever 32, andcomputes a control signal S for the bucket inflow control valve 208(i.e., a drive signal applied to a solenoid sector 208B thereof) basedon the illustrated table, followed by outputting the computed controlsignal. At this time, a maximum one of the boom-raising operation inputsignal X, the bucket-crowding operation input signal X, and thebucket-dumping operation input signal X from the control levers 32, 33is selected by the maximum value selector 235 and then inputted to thebypass drive signal processing unit 234. The bypass drive signalprocessing unit 234 computes a control signal S for the bypass flowcontrol valve 204 (i.e., a drive signal applied to a solenoid sector204B thereof) based on the illustrated table and outputs the computedcontrol signal.

On the other hand, the arm-recovery drive signal processing unit 252receives an arm-drawing operation input signal X from the control lever33, and computes a control signal S for the arm recovery flow controlvalve 224 (i.e., a drive signal applied to a solenoid sector 224Bthereof) based on the illustrated table, followed by outputting thecomputed control signal. Also, the bucket opening/closing recovery drivesignal processing unit 254 receives a bucket-closing operation inputsignal X from the control lever 34, and computes a control signal S forthe bucket opening/-closing recovery flow control valve 227 (i.e., adrive signal applied to a solenoid sector 227B thereof) based on theillustrated table, followed by outputting the computed control signal.

The operation of this embodiment thus constructed will be describedbelow, taking as an example the operations of boom lowering and armdrawing.

The loader type hydraulic excavator to which this embodiment is appliedoperates in a typical case as follows. From a condition where the frontoperating mechanism 14 is positioned close to the machine body 13 in afolded state, the boom-raising, arm-pushing and bucket-crowdingoperations are performed to scoop earth and sand in front of the frontoperating mechanism into the bucket 77. Then, the bucket 77 is elevatedto a high level immediately after the scooping, and the bucket openingportion 77B is opened relative to the bucket base portion 77A so thatthe earth and sand in the bucket 77 is released onto, e.g., alarge-sized dump truck. Thereafter, the front operating mechanism 14 isreturned to the initial folded state positioned close to the machinebody 13 through substantially simultaneous operations of not only bucketclosing and bucket dumping, but also boom lowering and arm drawing.

The features of this embodiment are typically usefully employed, inparticular, in the operations of boom lowering and arm drawing afterreleasing the scooped earth. These operations of boom lowering and armdrawing will be described below.

When the operator operates the control lever 32 in the directioncorresponding to the boom lowering with intent to lower the boom, forexample, after releasing the scooped earth, the produced operation inputsignal X is applied as a boom lowering command to the boom directionalflow control valves 10 c, 10 d, thus causing their spools to shift inthe corresponding directions. As a result, the hydraulic fluids from thehydraulic pumps 1 a, 1 b are supplied to the rod drawing-side chambers 5aB, 5 bB of the boom hydraulic cylinders 5 a, 5 b via the main line 115.

At that time, as in the above first embodiment, the return hydraulicfluids corresponding to a part (e.g., about ½) of the outflow rate fromthe rod pushing-side chambers 5 aA, 5 bA of the boom hydraulic cylindersare returned to the reservoir 2 from the rod pushing-side chambers 5 aA,5 bA thereof via the main line 105 and the meter-out throttles (notshown) of the directional flow control valves 10 c, 10 d.Simultaneously, the boom-recovery drive signal processing unit 251computes the drive signal S for the boom recovery flow control valve 221in accordance with the boom-lowering operation signal X from the controllever 32 and outputs the computed drive signal S to the solenoid sector221B of the boom recovery flow control valve 221. As a result, the boomrecovery flow control valve 221 is driven to the open side. On thisoccasion, because holding pressures are applied to the rod pushing-sidechambers 5 aA, 5 bA of the boom hydraulic cylinders 5 a, 5 b due to thedead load of the boom 75, the remaining part of the hydraulic fluidsfrom the rod pushing-side chambers 5 aA, 5 bA is introduced (recovered)to the rod drawing-side chambers 5 aB, 5 bB through the check valve 222and the boom recovery flow control valve 221 upon opening of the boomrecovery flow control valve 221.

Also, when the operator-operates the control lever 32 in the directioncorresponding to the arm drawing with intent to draw the arm, forexample, after releasing the scooped earth, the produced operation inputsignal X is applied as an arm drawing command to the arm directionalflow control valves 10 b, 10 e, thus causing their spools to shift inthe corresponding directions. As a result, the hydraulic fluids from thehydraulic pumps 1 a, 1 b are supplied to the rod drawing-side chamber 6Bof the arm hydraulic cylinder 6 via the main line 116.

At that time, as in the above case, the return hydraulic fluidcorresponding to a part (e.g., about ½) of the outflow rate from the rodpushing-side chamber 6A of the arm hydraulic cylinder is returned to thereservoir 2 from the rod pushing-side chamber 6A via the main line 106and the meter-out throttles (not shown) of the directional flow controlvalves 10 b, 10 e. Simultaneously, the arm-drawing drive signalprocessing unit 252 computes the drive signal S for the arm recoveryflow control valve 224 in accordance with the arm-drawing operationsignal X from the control lever 33 and outputs the computed drive signalS to the solenoid sector 227B of the arm recovery flow control valve224. As a result, the arm recovery flow control valve 224 is driven tothe open side. On this occasion, because a holding pressure is appliedto the rod pushing-side chamber 6A of the arm hydraulic cylinder 6 dueto the dead load of the arm 76, the remaining part of the hydraulicfluid from the rod pushing-side chamber 6A is introduced (recovered) tothe rod drawing-side chamber 6B through the check valve 225 and the armrecovery flow control valve 224 upon opening of the arm recovery flowcontrol valve 224.

With this embodiment thus constructed, as with the above fifthembodiment, when forming hydraulic fluid supply routes not passing thedirectional flow control valves 10 a-h to supply the hydraulic fluid ata large flow rate in a loader type hydraulic excavator of an super-largeclass, the branch line 150A leading to the rod pushing-side chambers 5aA, 5 bA of the boom hydraulic cylinders is first branched from thesupply line 100 serving as the common high-pressure line which isconnected the delivery sides of the hydraulic pumps 3 a, 3 b andextended to the side of the front operating mechanism 14. Then, theremaining part of the supply line 100 downstream of the position atwhich the branch line 150A is branched is constituted as the branch line150C leading to the rod pushing-side chamber 7A of the bucket hydrauliccylinder. Further, the boom inflow control valve 201 and the bucketinflow control valve 203 are disposed respectively in the branch lines150A, 150C to control the flows of the hydraulic fluids from the supplyline 100 to the hydraulic cylinders 5, 7.

When supplying the hydraulic fluids to the respective rod pushing-sidechambers 5 aA, 5 bA and 7A of the hydraulic cylinders 5, 7—to performthe boom-raising and bucket-crowding operations, in addition to theordinary supply of the hydraulic fluids to the respective rodpushing-side chambers 5 aA, 5 bA and 7A of the hydraulic cylinders 5, 7through the directional flow control valves 10 a-h, the hydraulic fluidsfrom the hydraulic pumps 3 a, 3 b are joined with the hydraulic fluids,which are supplied through the directional flow control valves 10 a-h,through the inflow control valves 201, 203 without passing thedirectional flow control valves 10 a-h. The joined hydraulic fluids arethen supplied to the respective rod pushing-side chambers 5 aA, 5 bA and7A of the hydraulic cylinders 5, 7. The return hydraulic fluids in thiscase are drained to the reservoir only via routes through thedirectional flow control valves 10 a-h.

Thus, in this embodiment, the hydraulic circuit is simplified asfollows. Regarding the inflow control valves, as in the fifth embodimentdescribed above, in consideration of the volume differences between therod pushing-side chambers 5 aA, 5 bA and the rod drawing-side chambers 5aB, 5 bB of the boom hydraulic cylinders 5 a, 5 b, only the inflowcontrol valve 201 in the branch line 150A associated with the rodpushing side (bottom side) is additionally provided to achieve thesupply of the hydraulic fluid at a large flow rate, while the inflowcontrol valves on the rod drawing side are omitted. For the buckethydraulic cylinder 7, unlike the fifth embodiment, the inflow controlvalve 208 for supplying the hydraulic fluid to the rod drawing-sidechamber 7B of the bucket hydraulic cylinder 7 is additionally provided.However, because the inflow control valve associated with the rodpushing side of the arm hydraulic cylinder 6 is omitted in considerationof the structure specific to the loader type hydraulic excavator asdescribed above, the total number of the inflow control valves is thesame. On the other hand, as described above, this embodiment realizesthe structure including no outflow control valves. As a result, thetotal number of the inflow and outflow control valves is greatly reducedfrom five (i.e., the flow control valves 201, 202, 203, 212 and 213) inthe fifth embodiment to three (i.e., the flow control valves 201, 203and 208). Correspondingly, the pressure loss caused by the flow controlvalves can be reduced. Also, since piping required for installation ofthe flow control valves is omitted and hence an accompanying pressureloss is eliminated, the pressure loss of the overall hydraulic drivesystem can be further reduced. In addition, with a reduction in thenumber of the flow control valves, it is possible to further simplifylayouts including routing of various pipes and arrangements of variousunits.

A seventh embodiment of the present invention will be described withreference to FIG. 13. This embodiment represents the case in which thepresent invention is applied to a loader type super-large-sizedhydraulic excavator of a class having a dead load of 800 tons, forexample, which is even larger than that described in the above sixthembodiment. Identical components to those in the above second and sixthembodiments are denoted by the same symbols, and a description of thosecomponents is omitted here as appropriate.

FIG. 13 is a hydraulic circuit diagram showing the overall constructionof a hydraulic drive system according to this embodiment.

Referring to FIG. 13, the hydraulic drive system of this embodimentcomprises eight hydraulic pumps 301 a, 301 b, 301 c, 301 d, 301 e, 301f, 303 a and 303 b driven by a not-shown first engine (prime mover) orsecond engine, boom hydraulic cylinders 305, 305, arm hydrauliccylinders 306, 306, bucket hydraulic cylinders 307, 307, bucketopening/closing hydraulic cylinders 308, 308, left and right travelhydraulic motors (not shown), and a swing hydraulic motor (not shown)which are supplied with hydraulic fluids delivered from the hydraulicpumps 301 a-f, 303 a and 303 b, and a hydraulic reservoir 302.

Of the hydraulic pumps 301 a-f, 303 a and 303 b, for example, thehydraulic pumps 301 a, 301 d, 301 e and 303 a are driven by the firstengine (not shown) disposed on the left side of a machine body 13, andthe hydraulic pumps 301 b, 301 c, 301 f and 303 b are driven by thesecond engine (not shown) disposed on the right side of the machine body13 (allocation of the hydraulic pumps with respect to the engines is notlimited to the above-described one, and may be set as appropriate inconsideration of horsepower distribution, etc.).

The hydraulic pump 301 a is connected to the left or right travelhydraulic motor, the boom hydraulic cylinders 305, 305, the armhydraulic cylinder 306, 306, and the bucket opening/closing hydrauliccylinders 308, 308 through a first travel directional flow control valve310 aa, a first boom directional flow control valve 310 ab, a first armdirectional flow control valve 310 ac, and a first bucketopening/closing directional flow control valve 310 ad, respectively.

The hydraulic pump 301 b is connected to the left or right travelhydraulic motor, the boom hydraulic cylinders 305, 305, rod pushing-sidechambers 307A, 307A of the bucket hydraulic cylinders 307, 307, rodpushing-side chambers 306A, 306A of the arm hydraulic cylinders 306,306, and the bucket hydraulic cylinders 307, 307 through a second traveldirectional flow control valve 310 ba, a second boom directional flowcontrol valve 310 bb, a first bucket-crowding/arm-pushing directionalflow control valve 310 bc, and a first bucket directional flow controlvalve 310 bd, respectively.

The hydraulic pump 301 c is connected to the left or right travelhydraulic motor, the boom hydraulic cylinders 305, 305, the armhydraulic cylinder 306, 306, and the bucket opening/closing hydrauliccylinders 308, 308 through a third travel directional flow control valve310 ca, a third boom directional flow control valve 310 cb, a second armdirectional flow control valve 310 cc, and a second bucketopening/closing directional flow control valve 310 cd, respectively.

The hydraulic pump 301 d is connected to the left or right travelhydraulic motor, rod pushing-side chambers 305A, 305A of the boomhydraulic cylinders 305, 305, the rod pushing-side chambers 307A, 307Aof the bucket hydraulic cylinders 307, 307, the rod pushing-sidechambers 306A, 306A of the arm hydraulic cylinders 306, 306, and thebucket hydraulic cylinders 307, 307 through a fourth travel directionalflow control valve 310 da, a first boom-raising directional flow controlvalve 310 db, a second bucket-crowding/arm-pushing directional flowcontrol valve 310 dc, and a second bucket directional flow control valve310 dd, respectively.

The hydraulic pump 301 e is connected to the swing hydraulic motor, therod pushing-side chambers 305A, 305A of the boom hydraulic cylinders305, 305, the rod pushing-side chambers 306A, 306A of the arm hydrauliccylinders 306, 306, and the rod pushing-side chambers 307A, 307A of thebucket hydraulic cylinders 307, 307 through a first swing directionalflow control valve 310 ea, a second boom-raising directional flowcontrol valve 310 eb, a first arm-pushing directional flow control valve310 ec, and a first bucket-crowding directional flow control valve 310ed, respectively.

The hydraulic pump 301 f is connected to the swing hydraulic motor, therod pushing-side chambers 305A, 305A of the boom hydraulic cylinders305, 305, the rod pushing-side chambers 306A, 306A of the arm hydrauliccylinders 306, 306, and the rod pushing-side chambers 307A, 307A of thebucket hydraulic cylinders 307, 307 through a second swing directionalflow control valve 310 fa, a third boom-raising directional flow controlvalve 310 fb, a second arm-pushing directional flow control valve 310fc, and a second bucket-crowding directional flow control valve 310 fd,respectively.

Those directional flow control valves 310 aa-fd are grouped into setseach comprising four valves to constitute a valve block percorresponding pump. More specifically, the directional flow controlvalves 310 aa, 310 ab, 310 ac and 310 ad associated with the hydraulicpump 301 a, the directional flow control valves 310 ba, 310 bb, 310 bcand 310 bd associated with the hydraulic pump 301 b, the directionalflow control valves 310 ca, 310 cb, 310 cc and 310 cd associated withthe hydraulic pump 301 c, the directional flow control valves 310 da,310 db, 310 dc and 310 dd associated with the hydraulic pump 301 d, thedirectional flow control valves 310 ea, 310 eb, 310 ec and 310 edassociated with the hydraulic pump 301 e, and the directional flowcontrol valves 310 fa, 310 fb, 310 fc and 310 fd associated with thehydraulic pump 301 f constitute valve blocks in one-to-one relation (sixsets in total).

The rod pushing-side chambers 305A, 305A of the boom hydraulic cylinders305, 305 are connected to the first to third boom directional flowcontrol valves 310 ab, 310 bb, 310 cb and the first to thirdboom-raising directional flow control valves 310 db, 310 eb, 310 fb viarespective main lines 405. Also, rod drawing-side chambers 305B, 305B ofthe boom hydraulic cylinders 305, 305 are connected to the first, secondand third boom directional flow control valves 310 ab, 310 bb and 310 cbvia respective main lines 415.

The rod pushing-side chambers 306A, 306A of the arm hydraulic cylinders306, 306 are connected to the first and second arm-pushing directionalflow control valves 310 ec, 310 fc and the first and secondbucket-crowding/arm-pushing directional flow control valves 310 bc, 310dc via respective main lines 406. Also, rod drawing-side chambers 306B,306B of the arm hydraulic cylinders 306, 306 are connected to the firstand second arm directional flow control valves 310 ac, 310 cc viarespective main lines 416.

The rod pushing-side chambers 307A, 307A of the bucket hydrauliccylinders 307, 307 are connected to the first and second bucketdirectional flow control valves 310 bd, 310 dd, the first and secondbucket-crowding directional flow control valves 310 ed, 310 fd, and thefirst and second bucket-crowding/arm-pushing directional flow controlvalves 310 bc, 310 dc via respective main lines 407. Rod drawing-sidechambers 307B, 307B of the bucket hydraulic cylinders 307, 307 areconnected to the first and second bucket directional flow control valves310 bd, 310 dd via respective main lines 417.

Rod pushing-side chambers 308A, 308A of the bucket opening/closinghydraulic cylinders 308, 308 are connected to the first and secondbucket opening/closing directional flow control valves 310 ad, 310 cdvia main lines 408. Rod drawing-side chambers 308B, 308B of the bucketopening/closing hydraulic cylinders 308, 308 are connected to the firstand second bucket opening/closing directional flow control valves 310ad, 310 cd via main lines 418.

The hydraulic pump 303 a is connected to the main lines 405, 407 and 417via a delivery line 402 a to which the hydraulic fluid delivered fromthe hydraulic pump 303 a is introduced, then via a supply line 400 aconnected at one side (left side as viewed in the drawing) thereof tothe delivery line 402 a, and then via branch lines 450A, 450B and 450Cbranched from the other side of the supply line 400 a.

In the branch lines 450A, 450B and 450C, there are disposed respectivelya boom inflow control valve 501 and bucket inflow control valves 502,503 which are each constituted as, e.g., a solenoid proportional valvewith a pressure compensating function and include respectively variablethrottles 501A, 502A and 503A for controlling flows of the hydraulicfluid supplied from the hydraulic pump 303 a to the rod pushing-sidechamber 305A of each boom hydraulic cylinder, the rod pushing-sidechamber 307A of each bucket hydraulic cylinder, and the rod drawing-sidechamber 307B of each bucket hydraulic cylinders to desired throttledrates. On the sides of the inflow control valve 501, 502 and 503 nearerto the hydraulic cylinders 305, 306 and 307, though not shown, checkvalves are disposed respectively which allow the hydraulic fluid to flowfrom the hydraulic pump 303 a to the rod pushing-side chamber 305A ofeach boom hydraulic cylinder and the rod pushing-side chamber 307A andthe rod drawing-side chamber 307B of each bucket hydraulic cylinder, butblock off the hydraulic fluid flowing in the reversed direction.

In this respect, a reservoir line 403 a is branched from the supply line400 a (or the delivery line 402 a as required). In this reservoir line403 a, a bypass flow control valve 504A is disposed which is constitutedas, e.g., a solenoid proportional valve with a pressure compensatingfunction and supplies the hydraulic fluid delivered from the hydraulicpump 303 a to the supply line 400 a through a variable throttle 504Aa ata desired flow rate while returning the remaining hydraulic fluid to thehydraulic reservoir 302 via the reservoir line 403 a. Additionally,though not shown, a relief valve is disposed between the delivery line402 a and the reservoir line 403 a to specify a maximum pressure in thesupply line 400 a serving as a high-pressure line.

Likewise, the hydraulic pump 303 b is connected to the main lines 405,407 and 417 via a delivery line 402 b to which the hydraulic fluiddelivered from the hydraulic pump 303 b is introduced, then via a supplyline 400 b connected at one side (left side as viewed in the drawing)thereof to the delivery line 402 b, and then via branch lines 451A, 451Band 451C branched from the other side of the supply line 400 b.

In the branch lines 451A, 451B and 451C, there are disposed respectivelya boom inflow control valve 505 and bucket inflow control valves 506,507 which are each constituted as, e.g., a solenoid proportional valvewith a pressure compensating function and include respectively variablethrottles 505A, 506A and 507A for controlling flows of the hydraulicfluid supplied from the hydraulic pump 303 b to the rod pushing-sidechamber 305A of each boom hydraulic cylinder, the rod pushing-sidechamber 307A of each bucket hydraulic cylinder, and the rod drawing-sidechamber 307B of each bucket hydraulic cylinders to desired throttledrates. On the sides of the inflow control valve 505, 506 and 507 nearerto the hydraulic cylinders 305, 306 and 307, though not shown, checkvalves are disposed respectively which allow the hydraulic fluid to flowfrom the hydraulic pump 303 b to the rod pushing-side chamber 305A ofeach boom hydraulic cylinder and the rod pushing-side chamber 307A andthe rod drawing-side chamber 307B of each bucket hydraulic cylinder, butblock off the hydraulic fluid flowing in the reversed direction.

In this respect, a reservoir line 403 b is branched from the supply line400 b (or the delivery line 402 b as required). In this reservoir line403 b, a bypass flow control valve 504B is disposed which is constitutedas, e.g., a solenoid proportional valve with a pressure compensatingfunction and supplies the hydraulic fluid delivered from the hydraulicpump 303 b to the supply line 400 b through a variable throttle 504Ba ata desired flow rate while returning the remaining hydraulic fluid to thehydraulic reservoir 302 via the reservoir line 403 b. Additionally,though not shown, a relief valve is disposed between the delivery line402 b and the reservoir line 403 b to specify a maximum pressure in thesupply line 400 b serving as a high-pressure line.

The hydraulic pumps 301 a-f, 303 a and 303 b, the directional flowcontrol valves 310 aa-fd, the delivery lines 402 a, 402 b, the reservoirlines 403 a, 403 b, the bypass flow control valves 504A, 504B, therelief valves, etc. are disposed in the machine body 13 of the hydraulicexcavator. The hydraulic cylinders 405, 406, 407 and 408, the supplylines 400 a, 400 b, the branch lines 450A-C, 451A-C, etc. are disposedon a front operating mechanism 14 of the hydraulic excavator.

As one of features of this embodiment, first, the connecting line 405connected to the rod pushing-side chambers 305A, 305A of the boomhydraulic cylinders 305, 305 and the connecting line 415 connected tothe rod drawing-side chambers 305B, 305B thereof are connected to eachother via a recovery line 520. In the recovery line 520, a boom recoveryflow control valve 521 is disposed which is constituted as, e.g., asolenoid proportional valve and includes a variable throttle forcontrolling the flows of the hydraulic fluids from the rod pushing-sidechambers 305A, 305A of the boom hydraulic cylinders 305, 305 to the roddrawing-side chambers 305B, 305B thereof to a desired throttled flowrate. Further, on the side of the boom recovery flow control valve 521nearer to the rod drawing-side chambers 305B, 305B, a check valve 522 isdisposed which allows the hydraulic fluids to flow from the rodpushing-side chambers 305A, 305A to the rod drawing-side chambers 305B,305B, but blocks off the hydraulic fluids from flowing in the reverseddirection. With such an arrangement, the hydraulic fluids in the rodpushing-side chambers 305A, 305A of the boom hydraulic cylinders 305,305 are introduced to the rod drawing-side chambers 305B, 305B.

Also, the connecting line 406 connected to the rod pushing-side chambers306A, 306A of the arm hydraulic cylinders 306, 306 and the connectingline 416 connected to the rod drawing-side chambers 306B, 306B thereofare connected to each other via a recovery line 523. In the recoveryline 523, an arm recovery flow control valve 524 is disposed which isconstituted as, e.g., a solenoid proportional valve and includes avariable throttle for controlling the flows of the hydraulic fluids fromthe rod pushing-side chambers 306A, 306A of the arm hydraulic cylinders306, 306 to the rod drawing-side chambers 306B, 306B thereof to adesired throttled flow rate. Further, on the side of the arm recoveryflow control valve 524 nearer to the rod drawing-side chambers 306B,306B, a check valve 525 is disposed which allows the hydraulic fluids toflow from the rod pushing-side chambers 306A, 306A to the roddrawing-side chambers 306B, 306B, but blocks off the hydraulic fluidsfrom flowing in the reversed direction. With such an arrangement, thehydraulic fluids in the rod pushing-side chambers 306A, 306A of the armhydraulic cylinders 306, 306 are introduced to the rod drawing-sidechambers 306B, 306B.

On the other hand, for the bucket opening/closing hydraulic cylinders308, 308, a circuit arrangement is added to provide a different recoveryfunction (operating in the reversed direction) from those for the boomhydraulic cylinders 305, 305 and the arm hydraulic cylinders 306, 306.More specifically, the connecting line 408 connected to the rodpushing-side chambers 308A, 308A of the bucket opening/closing hydrauliccylinders 308, 308 and the connecting line 418 connected to the roddrawing-side chambers 308B, 308B thereof are connected to each other viaa recovery line 526. In the recovery line 526, a bucket-opening/closingrecovery flow control valve 527 is disposed which is constituted as,e.g., a solenoid proportional valve and includes a variable throttle forcontrolling the flows of the hydraulic fluids from the rod drawing-sidechambers 308B of the bucket opening/closing hydraulic cylinders 308, 308to the rod pushing-side chambers 308A thereof to a desired throttledflow rate. Further, on the side of the bucket-opening/closing recoveryflow control valve 527 nearer to the rod pushing-side chambers 308B, acheck valve may be disposed which allows the hydraulic fluids to flowfrom the rod drawing-side chambers 308B to the rod pushing-side chambers308A, but blocks off the hydraulic fluids from flowing in the reverseddirection. With such an arrangement, the hydraulic fluid in the roddrawing-side chamber 308B of each bucket opening/closing hydrauliccylinder 308 is introduced to the rod pushing-side chamber 308A.

The other constructions and control procedures than described above,including the structure of the hydraulic excavator (except for outerdiameter dimensions, sizes, etc.) to which this embodiment is applied,are substantially the same as those in the sixth embodiment and hence adescription thereof is omitted here.

The operation of this embodiment thus constructed will be describedbelow, taking as an example the operations of boom lowering and armdrawing.

In the loader type hydraulic excavator to which this embodiment isapplied, as in the sixth embodiment, when the operator operates acontrol lever (not shown) in the direction corresponding to the boomlowering with intent to lower the boom, for example, after releasing thescooped earth, the produced operation input signal X is applied as aboom lowering command to the first to third boom directional flowcontrol valves 310 ab, 310 bb and 310 cb, thus causing their spools toshift in the corresponding directions. As a result, the hydraulic fluidsfrom the hydraulic pumps 301 a-c are supplied to the rod drawing-sidechambers 305B, 305B of the boom hydraulic cylinders 305, 305 via themain lines 415.

At that time, as in the above first and second embodiments, the returnhydraulic fluids corresponding to a part (e.g., about ½) of the outflowrate from the rod pushing-side chambers 305A, 305A of the boom hydrauliccylinders are returned to the reservoir 302 from the rod pushing-sidechambers 305A, 305A via the main lines 405 and respective meter-outthrottles (not shown) of the first to third boom directional flowcontrol valves 310 ab, 310 bb and 310 cb and the first to thirdboom-raising directional flow control valves 310 db, 310 eb and 310 fb.Simultaneously, a not-shown controller computes a drive signal S for theboom recovery flow control valve 521 in accordance with theboom-lowering operation signal X and outputs the computed drive signal Sto a solenoid sector of the boom recovery flow control valve 521. As aresult, the boom recovery flow control valve 521 is driven to the openside. On this occasion, because holding pressures are applied to the rodpushing-side chambers 305A, 305A of the boom hydraulic cylinders 305,305 due to the dead load of the boom, the remaining part of thehydraulic fluids from the rod pushing-side chambers 305A, 305A isintroduced (recovered) to the rod drawing-side chambers 305B, 305Bthrough the check valve 522 and the boom recovery flow control valve 521upon opening of the boom recovery flow control valve 521.

Also, when the operator operates a not-shown control lever in thedirection corresponding to the arm drawing with intent to draw the arm,for example, after releasing the scooped earth, the produced operationinput signal X is applied as an arm drawing command to the first andsecond arm directional flow control valves 310 ac, 310 cc, thus causingtheir spools to shift in the corresponding directions. As a result, thehydraulic fluids from the hydraulic pumps 301 a, 301 c are supplied tothe rod drawing-side chambers 306B, 306B of the arm hydraulic cylinders306, 306 via the main lines 416.

At that time, as in the above case, the return hydraulic fluidscorresponding to a part (e.g., about ½) of the outflow rate from the rodpushing-side chambers 306A, 306A of the arm hydraulic cylinders arereturned to the reservoir 302 from the rod pushing-side chambers 306Avia the main lines 406 and respective meter-out throttles (not shown) ofthe first and second arm directional flow control valves 310 ac, 310 cc,the first and second arm-pushing directional flow control valves 310 ec,310 fc, and the first and second bucket-crowding/arm-pushing directionalflow control valves 310 bc, 310 dc. Simultaneously, a not-showncontroller computes a drive signal S for the arm recovery flow controlvalve 524 in accordance with the arm-drawing operation signal X from thecontrol lever and outputs the computed drive signal S to a solenoidsector of the arm recovery flow control valve 524. As a result, the armrecovery flow control valve 524 is driven to the open side. On thisoccasion, because a holding pressure is applied to the rod pushing-sidechamber 306A of each arm hydraulic cylinder 306 due to the dead load ofthe arm, the remaining part of the hydraulic fluid drained from the rodpushing-side chamber 306A is introduced (recovered) to the roddrawing-side chamber 306B through the check valve 525 and the armrecovery flow control valve 524 upon opening of the arm recovery flowcontrol valve 524.

With this embodiment thus constructed, as with the above sixthembodiment, the number of the flow control valves is reduced, thusresulting in the advantages of a reduction in the pressure loss of theoverall hydraulic drive system and simplified layouts therein.

Also, a total flow rate of the return hydraulic fluids from the rodpushing-side chambers 305A, 305A of the boom hydraulic cylinders 305,305 during the boom-lowering operation is accommodated as a flow rateordinarily drained to the reservoir 302 through the meter-out throttlesof the directional flow control valves 310 ab, 310 bb, 310 cb, 310 db,310 eb and 310 fb and a flow rate recovered to the rod drawing-sidechambers 305B, 305B through the boom recovery flow control valve 521.Further, a total flow rate of the return hydraulic fluids from the rodpushing-side chambers 306A, 306A of the arm hydraulic cylinders 306, 306during the arm-drawing operation is accommodated as a flow rateordinarily drained to the reservoir 302 through the meter-out throttlesof the directional flow control valves 310 ac, 310 bc, 310 cc, 310 dc,310 ec and 310 fc and a flow rate recovered to the rod drawing-sidechambers 306B through the arm recovery flow control valve 524. With suchan arrangement, regarding the boom hydraulic cylinders 305, 305 and thearm hydraulic cylinders 306, 306, parts of the return hydraulic fluids(extra flows to be drained) from the rod drawing-side chambers 305B,305B and the rod drawing-side chambers 306B, 306B are each effectivelyutilized as a recovery flow. It is therefore possible to, as with thesixth embodiment, omit an outflow control valve having a large capacityand an associated outflow line adapted for a large flow rate, which areeach otherwise provided in association with the boom hydraulic cylinders305, 305 and the arm hydraulic cylinders 306, 306, and hence tosufficiently increase the energy efficiency.

The flow control valves 201, 202, 203, 208, 501, 502, 503, 505, 506 and507 described in the above first to seventh embodiments may be eachconstituted as a seat valve having a relatively small pressure loss. Anexample of the construction of such a seat valve will be described belowwith reference to FIGS. 14 and 15. FIG. 14 shows the flow control valve202 as one example extracted from the flow control valves shown in FIG.1, and FIG. 15 shows the structure of the seat valve corresponding tothe diagram shown in FIG. 14.

More specifically, in FIG. 15, a main valve (seat valve) 603 constitutedby a poppet fitted into a casing 602 has a seat portion 603A forestablishing and cutting off communication between an inlet line 621communicating with the supply line 100 and an outlet line 631 connectedto the branch line 150B through the check valve, an end face 603Csubjected to a pressure in the outlet line 631, an end face 603Bpositioned on the opposite side to the end face 603C and subjected to apressure in a back pressure chamber 604 formed between the casing 602and the outlet line 603B, and a throttle slit 603D for communicating theinlet line 621 and the back pressure chamber 604 with each other.Further, a pilot line 605 for communicating the back pressure chamber604 and the outlet line 631 with each other is formed in the casing 602.Midway the pilot line 605, a control valve (variable throttle) 606 forcontrolling a control pressure is disposed which is constituted as,e.g., a proportional solenoid valve for adjusting a flow rate in thepilot line 605 in accordance with a command signal 601 from thecontroller.

In the arrangement described above, the pressure in the inlet line 621is introduced to the back pressure chamber 604 through the throttle slit603D, and under the action of this introduced pressure, the main valve603 is pressed downward as viewed in the drawing so that thecommunication between the inlet line 621 and the outlet line 631 is cutoff by the main valve abutting against the seat portion 603A. In thatcondition, when the desired command signal 601 is applied to a solenoiddriving sector 606 a of the control valve 606 to open the control valve606, the fluid in the inlet line 621 is caused to flow out to the outletline 631 through the throttle slit 603D, the back pressure chamber 604,the control valve 606, and the pilot line 605. Such an outflow lowersthe pressure in the back pressure chamber 604 with the throttlingeffects of both the throttle slit 603D and the control valve 606,whereby forces acting upon the end face 603A and an end face 603E becomelarger than forces acting upon the end face 603B. As a result, the mainvalve 603 is moved upward as viewed in the drawing, thus allowing thefluid in the inlet line 621 to flow out to the outlet line 631. In thisrespect, if the main valve 603 is moved upward through an excessivestroke, the throttle opening of the throttle slit 603D is increased andthe pressure in the back pressure chamber 604 rises, whereby the mainvalve 603 is moved downward as viewed in the drawing.

In such a way, the main valve 603 is stopped at a position where thethrottle opening of the throttle slit 603D is in balance with thethrottle opening of the control valve 606. Accordingly, the flow rate ofthe fluid from the inlet line 621 to the outlet line 631 can becontrolled as desired in accordance with the command signal 601.

It is needless to say that the flow control valves (i.e., the flowcontrol valves not required to have the function of a check valve) 204,211, 212 and 213 other than the above-mentioned ones or the recoveryflow control valves 221, 224, 227, 521, 524 and 527 can also be eachconstituted as a similar seat valve.

In particular, preferably, each flow control valve is arranged such thatan axis k (see FIG. 15) of the main valve 603 lies substantially in thehorizontal direction. In FIGS. 2 and 5 representing the first embodimentand the second embodiment, respectively, the direction of the axis k isshown, by way of example, in the valve unit 190 in which the flowcontrol valves 201 to 203, the outflow control valves 211 to 213, etc.are disposed (this is similarly applied to the valve unit 190′). Thatarrangement results in the following advantage. In FIGS. 2 and 5, withthe direction of the axis k being substantially horizontal as shown,when the front operating mechanism 14 is operated to rotate in the planedirection of the drawing sheets, acceleration generated by the rotationof the front operating mechanism is directed perpendicularly to thedirection in which the main valve 603 is moved to open and close, sothat the valve opening and closing operations are not adversely affectedby the generated acceleration. It is hence possible to ensure the smoothand reliable opening and closing operations of the main valve 603.

While, in the above description, the command signal is applied to thesolenoid driving sector 606A of the control valve 606, which is asolenoid proportional valve, to shift the control valve 606 forproducing a pilot pressure as the control pressure directly in the pilotline 605, the present invention is not limited to such an arrangement.For example, when the main valve 603 has a large size and a relativelyhigh pilot pressure is required to drive the main valve 603, a hydraulicpilot selector valve for producing a secondary pilot pressure may beadditionally provided. In this case, the selector valve is shifted undera primary pilot pressure produced by the control valve 606 to producethe secondary pilot pressure higher than the primary pilot pressurebased on an original pilot pressure from a hydraulic source, and thethus-produced secondary pilot pressure is introduced, as the controlpressure, to the main valve 603, thereby shifting the main valve 603.

Furthermore, while the first to seventh embodiments represent the casein which the present invention is applied to a hydraulic excavator, thepresent invention is also widely applicable to other variousconstruction machines each having a swing body, a travel body, and afront operating mechanism.

INDUSTRIAL APPLICABILITY

According to the present invention, the number of flow control valvesand the length of piping required for connection of the flow controlvalves can be further cut, and a total pressure loss can be furtherreduced. Thus, it is also possible to simplify layouts of hydraulicpiping between hydraulic sources and actuators.

1. A hydraulic drive system for a construction machine comprising atravel body, a swing body swingably mounted onto said travel body, and amulti-articulated front operating mechanism coupled to said swing bodyin a vertically angularly movable manner and made up of a boom and abucket, wherein said hydraulic drive system comprises: a first hydraulicpump and a second hydraulic pump driven by prime movers; a plurality ofhydraulic cylinders including a boom hydraulic cylinder, an armhydraulic cylinder, and a bucket hydraulic cylinder supplied withhydraulic fluids delivered from said first hydraulic pump and saidsecond hydraulic pump to drive said boom, said arm, and said bucket,respectively: a plurality of directional flow control valves forcontrolling respective flows of the hydraulic fluid supplied from saidfirst hydraulic pump to said plurality of hydraulic cylinders; a commonhigh-pressure line having one side connected to the delivery side ofsaid second hydraulic pump and the other side extended to the side ofsaid front operating mechanism; a boom branch line branched from saidcommon high-pressure line and connected on the side opposite to thebranched side to a rod pushing-side chamber of said boom hydrauliccylinder; a boom inflow control valve disposed near a branch position atwhich said boom branch line is branched from said common high-pressureline, and controlling a flow of a hydraulic fluid supplied from saidcommon high-pressure line to the rod pushing-side chamber of said boomhydraulic cylinder; an arm branch line branched from said commonhigh-pressure line at a position downstream of the branch position ofsaid boom branch line and connected on the side opposite to the branchedside to a rod pushing-side chamber of said arm hydraulic cylinder; anarm inflow control valve disposed near a branch position at which saidarm branch line is branched from said common high-pressure line, andcontrolling a flow of a hydraulic fluid supplied from said commonhigh-pressure line to the rod pushing-side chamber of said arm hydrauliccylinder; a bucket branch line branched from said common high-pressureline at a position downstream of the branch position of said boom branchline and connected on the side opposite to the branched side to a rodpushing-side chamber of said bucket hydraulic cylinder; and a bucketinflow control valve disposed near the branch position (D2) at whichsaid bucket branch line is branched from said common high-pressure line,and controlling a flow of a hydraulic fluid supplied from said commonhigh-pressure line to the rod pushing-side chamber of said buckethydraulic cylinder, said hydraulic system being configured such thatwhen said boom hydraulic cylinder, said arm hydraulic cylinder and saidbucket hydraulic cylinder are operated to extend, the hydraulic fluidfrom said first hydraulic pump is supplied to said rod pushing-sidechambers of said boom, arm and bucket hydraulic cylinders through saidplurality of directional flow control valves, respectively, and thehydraulic fluid from said second hydraulic pump is supplied to said rodpushing-side chambers of said boom, arm and bucket hydraulic cylindersthrough said common high-pressure line, said boom branch line, said armbranch line and said bucket branch line and further said boom inflowcontrol valve, said arm inflow control valve and said bucket inflowcontrol valve, respectively, and being joined with the hydraulic fluidfrom said first hydraulic pump, while the fluids discharged from the roddrawing-side chambers of said boom, arm and bucket hydraulic cylindersare returned to said reservoir only through said plurality ofdirectional flow control valves, respectively, wherein: said inflowcontrol valves are all disposed together in one control valve unit.
 2. Ahydraulic drive system for a construction machine according to claim 1,wherein: said one control valve unit is disposed on said boom.
 3. Ahydraulic drive system for a construction machine comprising a travelbody, a swing body swingably mounted onto said travel body, and amulti-articulated front operating mechanism coupled to said swing bodyin a vertically angularly movable manner and made up of a boom and abucket, wherein said hydraulic drive system comprises: a first hydraulicpump and a second hydraulic pump driven by prime movers; a plurality ofhydraulic cylinders including a boom hydraulic cylinder, an armhydraulic cylinder, and a bucket hydraulic cylinder supplied withhydraulic fluids delivered from said first hydraulic pump and saidsecond hydraulic pump to drive said boom, said arm, and said bucket,respectively; a plurality of directional flow control valves forcontrolling respective flows of the hydraulic fluid supplied from saidfirst hydraulic pump to said plurality of hydraulic cylinders; a commonhigh-pressure line having one side connected to the delivery side ofsaid second hydraulic pump and the other side extended to the side ofsaid front operating mechanism; a boom branch line branched from saidcommon high-pressure line and connected on the side opposite to thebranched side to a rod pushing-side chamber of said boom hydrauliccylinder; a boom inflow control valve disposed near a branch position atwhich said boom branch line is branched from said common high-pressureline, and controlling a flow of a hydraulic fluid supplied from saidcommon high-pressure line to the rod pushing-side chamber of said boomhydraulic cylinder; an arm branch line branched from said commonhigh-pressure line at a position downstream of the branch position ofsaid boom branch line and connected on the side opposite to the branchedside to a rod pushing-side chamber of said arm hydraulic cylinder; anarm inflow control valve disposed near a branch position at which saidarm branch line is branched from said common high-pressure line, andcontrolling a flow of a hydraulic fluid supplied from said commonhigh-pressure line to the rod pushing-side chamber of said arm hydrauliccylinder; a bucket branch line branched from said common high-pressureline at a position downstream of the branch position of said boom branchline and connected on the side opposite to the branched side to a rodpushing-side chamber of said bucket hydraulic cylinder; and a bucketinflow control valve disposed near the branch position (D2) at whichsaid bucket branch line is branched from said common high-pressure line,and controlling a flow of a hydraulic fluid supplied from said commonhigh-pressure line to the rod pushing-side chamber of said buckethydraulic cylinder, said hydraulic system being configured such thatwhen said boom hydraulic cylinder, said arm hydraulic cylinder and saidbucket hydraulic cylinder are operated to extend, the hydraulic fluidfrom said first hydraulic pump is supplied to said rod pushing-sidechambers of said boom, arm and bucket hydraulic cylinders through saidplurality of directional flow control valves, respectively, and thehydraulic fluid from said second hydraulic pump is supplied to said rodpushing-side chambers of said boom, arm and bucket hydraulic cylindersthrough said common high-pressure line, said boom branch line, said armbranch line and said bucket branch line and further said boom inflowcontrol valve, said arm inflow control valve and said bucket inflowcontrol valve, respectively, and being joined with the hydraulic fluidfrom said first hydraulic pump, while the fluids discharged from the roddrawing-side chambers of said boom, arm and bucket hydraulic cylindersare returned to said reservoir only through said plurality ofdirectional flow control valves, respectively, wherein said hydraulicdrive system further comprises at least one of three sets comprising: aboom return fluid joining line branched from said boom branch line at aposition nearer to said boom hydraulic cylinder than said boom inflowcontrol valve and connected on the side opposite to the branched side toa hydraulic reservoir, and a boom outflow control valve disposed in saidboom return fluid joining line near a branch position at which said boomreturn fluid joining line is branched from said boom branch line andcontrolling a flow of a hydraulic fluid drained from said boom hydrauliccylinder to said hydraulic reservoir; an arm return fluid joining linebranched from said arm branch line at a position nearer to said armhydraulic cylinder than said arm inflow control valve and connected onthe side opposite to the branched side to said hydraulic reservoir, andan arm outflow control valve disposed in said arm return fluid joiningline near a branch position at which said arm return fluid joining lineis branched from said arm branch line and controlling a flow of ahydraulic fluid drained from said arm hydraulic cylinder to saidhydraulic reservoir; and a bucket return fluid joining line branchedfrom said bucket branch line at a position nearer to said buckethydraulic cylinder than said bucket inflow control valve and connectedon the side opposite to the branched side to said hydraulic reservoir,and a bucket outflow control valve disposed in said bucket return fluidjoining line near a branch position at which said bucket return fluidjoining line is branched from said bucket branch line and controlling aflow of a hydraulic fluid drained from said bucket hydraulic cylinder tosaid hydraulic reservoir.
 4. A hydraulic drive system for a constructionmachine according to claim 3, wherein: said inflow control valves andsaid outflow control valves are all disposed together in one controlvalve unit.
 5. A hydraulic drive system for a construction machine,wherein said hydraulic drive system comprises: a first hydraulic pumpand a second hydraulic pump driven by prime movers; a plurality ofhydraulic cylinders driven by hydraulic fluids delivered from said firsthydraulic pump and said second hydraulic pump; a plurality ofdirectional flow control valves for controlling respective flows of thehydraulic fluid supplied from said first hydraulic pump to saidplurality of hydraulic cylinders; at least one inflow control valve forcontrolling a flow of the hydraulic fluid delivered from said secondhydraulic pump and supplied to at least one rod pushing-side chamberamong said plurality of hydraulic cylinders without passing saiddirectional flow control valves; a bypass flow control valve forreturning the hydraulic fluid delivered from said second hydraulic pumpto a reservoir; a recovery flow control valve for introducing thehydraulic fluid in at least one rod pushing-side chamber among saidplurality of hydraulic cylinders to a rod drawing-side chamber thereof;said hydraulic system being configured such that when said plurality ofhydraulic cylinders are operated to extend, the hydraulic fluid fromsaid first hydraulic pump is supplied to said rod pushing-side chambersof said plurality of hydraulic cylinders through said plurality ofdirectional flow control valves, respectively, and the hydraulic fluidfrom said second hydraulic pump is supplied to said rod pushing-sidechambers of said plurality of hydraulic cylinders through said pluralityof inflow control valves respectively, and being joined with thehydraulic fluid from said first hydraulic pump, while the fluidsdischarged from the rod drawing-side chambers of said plurality ofhydraulic cylinders are returned to said reservoir only through saidplurality of directional flow control valves, respectively, and whensaid plurality of hydraulic cylinders are operated to contract, thehydraulic fluid from said first hydraulic pump is supplied to said roddrawing-side chambers of said plurality of hydraulic cylinders throughsaid plurality of directional flow control valves, respectively, andpart of the hydraulic fluid from said at least one rod pushing-sidechamber amond said plurality of hydraulic cylinders is supplied to therod drawing-side chamber of the corresponding hydraulic cylinder, whilethe remaining part of the fluid discharged from the rod pushing-sidechamber of the corresponding hyraulic cylinder and the fluids dischargedfrom the rold pushing-side chambers of the other hydraulic cylinders arereturned to said reservoir only through said plurality of directionalflow control valves, respectively.
 6. A hydraulic drive system for aconstruction machine according to claim 5, wherein check valves aredisposed respectively in branch lines for supplying the hydraulic fluidto the rod pushing-side chambers of said hydraulic cylinders.
 7. Ahydraulic drive system for a construction machine according to claim 5,wherein: at least one of said inflow control valves, said outflowcontrol valves, and said bypass flow control valves is constituted as aseat valve.
 8. A hydraulic drive system for a construction machineaccording to claim 7, wherein: said seat valve is arranged such that anaxis (k) thereof lies substantially in the horizontal direction.
 9. Ahydraulic drive system for a construction machine comprising a travelbody, a swing body swingably mounted onto said travel body, and amulti-articulated front operating mechanism coupled to said swing bodyin a vertically angularly movable manner and made up of a boom, an armand a bucket, wherein said hydraulic drive system comprises: a firsthydraulic pump and a second hydraulic pump driven by prime movers; aplurality of hydraulic cylinders including a boom hydraulic cylinder, anarm hydraulic cylinder and a bucket hydraulic cylinder supplied withhydraulic fluids delivered from said first hydraulic pump and saidsecond hydraulic pump to drive said boom, said arm, and said bucket,respectively; a plurality of directional flow control valves forcontrolling respective flows of the hydraulic fluid supplied from saidfirst hydraulic pump to said plurality of hydraulic cylinders; at leastone inflow control valve for controlling a flow of the hydraulic fluiddelivered from said second hydraulic pump and supplied to a rodpushing-side chamber of at least said boom hydraulic cylinder among saidplurality of hydraulic cylinders without passing said directional flowcontrol valves; a bypass flow control valve for returning the hydraulicfluid delivered from said second hydraulic pump to a reservoir; at leastone recovery flow control valve for introducing the hydraulic fluid inthe rod pushing-side chamber of at least said boom hydraulic cylinderamong said plurality of hydraulic cylinders to a rod drawing-sidechamber thereof; and said hydraulic system being configured such thatwhen said boom hydraulic cylinder is operated to extend, the hydraulicfluid from said first hydraulic pump is supplied to said rodpushing-side chamber of said boom hydraulic cylinder through thedirectional flow control valve for the boom hydraulic cylinder, and thehydraulic fluid from said second hydraulic pump is supplied to said rodpushing-side chamber of said boom hydraulic cylinder through said inflowcontrol valve, and being joined with the hydraulic fluid from said firsthydraulic pump, while the fluid discharged from the rod drawing-sidechamber of said boom hydraulic cylinder is returned to said reservoironly throught the directional flow control valve for the boom hydrauliccylinder, and when said boom hydraulic cylinder is operatad to contract,the hydraulic fluid from said first hydraulic pump is supplied to saidrod drawing-side chamber of said boom hydraulic cylinder through thedirectional flow control valve for the boom hydraulic cylinder, and partof the hydraulic fluid from said rod pushing-side chamber of the boomhydraulic cylinder is supplied to the rod drawing-side chamber of theboom hydraulic cylinder through said recovery flow control valve, whilethe remaining part of the fluid discharged from the rod pushing-sidechamber of the boom hydraulic cylinder is returned to said reservoironly through the directional flow control valve for the boom hydrauliccylinder.
 10. A hydraulic drive system for a construction machineaccording to claim 9, wherein: said inflow control valves are alldisposed together in one control valve unit.
 11. A hydraulic drivesystem for a construction machine comprising a travel body, a swing bodyswingably mounted onto said travel body, and a multi-articulated frontoperating mechanism made up of a boom rotatably coupled to saidswingbody,an arm rotatably coupled to said boom, and a bucket rotatablycoupled to said arm to be open forward in a ground contact state,wherein said hydraulic drive system comprises: at least one firsthydraulic pump and at least one second hydraulic pump driven by aplurality of prime movers; a plurality of hydraulic cylinders includinga boom hydraulic cylinder, an arm hydraulic cylinder and a buckethydraulic cylinder supplied with hydraulic fluids delivered from saidfirst hydraulic pump and said second hydraulic pump to drive said boom,said arm and said bucket, respectively, and an opening/closing hydrauliccylinder supplied with the hydraulic fluids to open and close saidbucket; a plurality of directional flow control valves for controllingrespective flows of the hydraulic fluid supplied from said firsthydraulic pump to said plurality of hydraulic cylinders; a boom-raisinginflow control valve, a bucket-crowding inflow control valve and abucket-dumping inflow control valve for controlling respective flows ofthe hydraulic fluid delivered from said second hydraulic pump andsupplied to rod pushing-side chamber of said boom hydraulic cylinder, arod pushing-side chamber of said bucket hydraulic clyinder, and a roddrawing-side chamber of said bucket hydraulic cylinder without passingsaid directional flow control valves; a bypass flow control valve forreturning the hydraulic fluid delivered from said second hydraulic pumpto a reservoir; at least two recovery flow control valve for introducingthe hydraulic fluids in the rod pushing-side chambers of at least saidboom hydraulic cylinder and said arm hydraulic cylinder among saidplurality of hydraulic cylinders to rod drawing-side chambers thereof;and said hydraulic system being configured such that when said boomhydraulic cylinder, said arm hydraulic cylinder and said buckethydraulic cylinder are operated to extend as far as the boom and buckethydraulic cylinders are concerned, the hydraulic fluid from said firsthydraulic pump is supplied to said rod pushing-side chambers thereofthrough the directional flow control valves for the boom and buckethydraulic cylinders, respectively, and the hydraulic fluid from saidsecond hydraulic pump is supplied to said rod pushing-side chambersthereof through said boom-raising and bucket-crowding inflow controlvalves, respectively, and being joined with the hydraulic fluid fromsaid first hydraulic pump, and with respect to the arm hydrauliccylinder, the hydraulic fluid from said first hydraulic pump is suppliedto said rod pushing-side chamber thereof through the directional flowcontrol valve for the arm hydraulic cylinder, while the fluidsdischarged from the rod drawing-side chambers of said boom arm andbucket hydraulic cylinders are returned to said reservoir only throughthe directional flow control valves for the boom, arm and buckethydraulic cylinders, and when said boom hydraulic cylinder, said armhydraulic cylinder and said bucket hydraulic cylinder are operated tocontract, and with respect to said boom and arm hydraulic cylinders, thehydraulic fluid from said first hydraulic pump is supplied to said roddrawing-side chambers thereof through the directional flow controlvalves for the boom and arm hydraulic cylinders, respectively, and partof the hydraulic fluids from said rod pushing-side chambers thereof aresupplied to the rod drawing-side chambers thereof through said recoveryflow control valves and with respect to said bucket hydraulic cylinder,the hydraulic fluid from said first hydraulic pump is supplied to saidrod drawing-side chamber thereof through the directional flow controlvalve for the bucket hydraulic cylinder, and the hydraulic fluid fromsaid second hydraulic pump is supplied to the rod drawing-side chamberthereof through said bucket-dumping inflow control valve, and beingjoined with the hydraulic fluid from the first hydraulic pump, while theremaining part of the fluid discharged from the rod pushing-sidechambers of the boom and arm hydraulic cylinders and all fluiddischarged from the rod pushing-side chamber of the bucket hydrauliccylinder are returned to said reservoir only through the directionalflow control valves for the boom, arm and bucket hydraulic cylinders.12. A hydraulic drive system for a construction machine comprising atravel body, a swing body swingably mounted onto said travel body, and amulti-articulated front operating mechanism made up of a boom rotatablycoupled to said swing body, an arm rotatably coupled to said boom, and abucket rotatably coupled to said arm to be open rearward in a groundcontact state, wherein said hydraulic drive system comprises: at leastone first hydraulic pump and at least one second hydraulic pump drivenby a plurality of prime movers; a plurality of hydraulic cylindersincluding a boom hydraulic cylinder, an arm hydraulic cylinder and abucket hydraulic cylinder supplied with hydraulic fluids delivered fromsaid first hydraulic pump and said second hydraulic pump to drive saidboom, said arm and said bucket, respectively; a plurality of directionalflow control valves for controlling respective flows of the hydraulicfluid supplied from said first hydraulic pump to said plurality ofhydraulic cylinders; a plurality of inflow control valve for controllingrespective flows of the hydraulic fluid delivered from said secondhydraulic pump and supplied to rod pushing-side chambers of said boomhydraulic cylinders, said arm hydraulic cylinder and said buckethydraulic cylinder without passing said directional flow control valves;a bypass flow control valve for returning the hydraulic fluid deliveredfrom said second hydraulic pump to a reservoir; at least one recoveryflow control valve for introducing the hydraulic fluid in the rodpushing-side chamber of at least said boom hydraulic cylinder among saidplurality of hydraulic cylinders to a rod drawing-side chamber thereof;and said hydraulic system being configured such that when said boomhydraulic cylinder, said arm hydraulic cylinder and said buckethydraulic cylinder are operated to extend, the hydraulic fluid from saidfirst hydraulic pump is supplied to said rod pushing-side chambers ofsaid boom, arm and bucket hydraulic cylinders through the directionalflow control valves for the boom, arm and bucket hydraulic cylinders,respectively, and the hydraulic fluid from said second hydraulic pump issupplied to said rod pushing-side chambers thereof through said inflowcontrol valves, respectively, and being joined with the hydraulic fluidfrom said first hydraulic pump, while the fluids discharged from the roddrawing-side chambers of said boom, arm and bucket hydraulic cylindersare returned to said reservoir only through the directional flow controlvalves for the boom, arm and bucket hydraulic cylinders, and when saidboom hydraulic cylinder among said plurality of hydraulic cylinders isoperated to contract the hydraulic fluid from said first hydraulic pumpis supplied to said rod drawing-side clamber thereof through thedirectional flow control valve for the boom hydraulic cylinder, and partof the hydraulic fluid from said rod pushing-side chamber thereof issupplied to the rod drawing-side chamber thereof through said recoveryflow control valve, while the remaining part of the fluid dischargedfrom the rod pushing-side chamber thereof is returned to said reservoironly through the directional flow control valve for the boom hydrauliccylinders.
 13. A hydraulic drive system for a construction machinecomprising a travel body, a swing body swingably mounted onto saidtravel body, and a multi-articulated front operating mechanism made upof a boom rotatably coupled to said swing body, an arm rotatably coupledto said boom, and a bucket rotatably coupled to said arm to be openforward in a ground contact state, wherein said hydraulic drive systemcomprises: six first hydraulic pumps and two second hydraulic pumpsdriven by a plurality of prime movers; a boom hydraulic cylinder, an armhydraulic cylinder and a bucket hydraulic cylinder supplied withhydraulic fluids delivered from said first hydraulic pump and saidsecond hydraulic pump to drive said boom, said arm and said bucket,respectively, and an opening/closing hydraulic cylinder supplied withthe hydraulic fluids to open and close said bucket; a plurality of boomdirectional flow control valves, a plurality of arm directional flowcontrol valves, a plurality of bucket directional flow control valves,and a plurality of opening/closing directional flow control valves forcontrolling respective flows of the hydraulic fluids supplied from saidsix first hydraulic pumps to said boom hydraulic cylinder, said armhydraulic cylinder, said bucket hydraulic cylinder, and saidopening/closing hydraulic cylinder; a boom-raising inflow control valve,a bucket-crowding inflow control valve and a bucket-dumping inflowcontrol valve for controlling respective flows of the hydraulic fluidsdelivered from said two second hydraulic pumps and supplied to a rodpushing-side chamber of said boom hydraulic cylinder, a rod pushing-sidechamber of said bucket hydraulic cylinder, and a rod drawing-sidechamber of said bucket hydraulic cylinder without passing said pluralityof boom directional flow control valves and said plurality of bucketdirectional flow control valves; a bypass flow control valve forreturning the hydraulic fluids delivered from said two second hydraulicpumps to a reservoir; a boom recovery flow control valve and an armrecovery flow control valve for introducing the hydraulic fluids in therod pushing-side chambers of said boom hydraulic cylinder and said armhydraulic cylinder to rod drawing-side chambers thereof; anopening/closing recovery flow control valve for introducing thehydraulic fluid in a rod drawing-side chamber of said opening/closinghydraulic cylinder to a rod pushing-side chamber thereof; and saidhydraulic system being configured such that when said boom hydrauliccylinder, said arm hydraulic cylinder, said bucket hydraulic cylinderand said opening/closing hydraulic cylinder are operated to extend, andwith respect to the boom and bucket hydraulic cylinders, the hydraulicfluids from said first hydraulic pumps are supplied to said rodpushing-side chambers thereof through the directional flow controlvalves for the boom and bucket hydraulic cylinders, respectively, andthe hydraulic fluids from said second hydraulic pumps are supplied tosaid rod pushing-sides chambers thereof through said boom-raising andbucket-crowding inflow control valves, respectively, with being joinedwith the hydraulic fluids from said first hydraulic pumps, and withrespect to the arm hydraulic cylinder, the hydraulic fluids from saidfirst hydraulic pumps are supplied to said rod pushing-side chamberthereof through the directional flow control valves for the armhydraulic cylinder, and with respect to the opening/closing hydrauliccylinder, the hydraulic fluids from said first hydraulic pumps aresupplied to said rod pushing-side chamber thereof through thedirectional flow control valves for the opening/closing hydrauliccylinder, and part of the fluid discharged from said rod drawing-sidechamber thereof is supplied to said rod pushing-side chamber thereofthrough said opening/closing recovery flow control valve, while allfluids discharged from the rod drawing-side chambers of said boom, armand bucket hydraulic cylinders and the remaining part of the fluiddischarged from the rod pushing-side chamber of the opening/closing,hydraulic cylinder are returned to said reservoir only through thedirectional flow control valves for the boom, arm, bucket andopening/closing hydraulic cylinders, and when said boom hydrauliccylinder, said arm hydraulic cylinder, said bucket hydraulic cylinderand said opening/closing hydraulic cylinder are operated to contract,and with respect to said boom and arm hydraulic cylinders, the hydraulicfluids from said first hydraulic pumps are supplied to said roddrawing-side chambers thereof through the directional flow controlvalves for the boom and arm hydraulic cylinders, respectively, and partof the hydraulic fluids from said rod pushing-side chambers thereof aresupplied to the rod drawing-side chambers thereof through said boom andarm recovery flow control valves, and with respect to said buckethydraulic cylinder, the hydraulic fluids from said first hydraulic pumpsare supplied to said rod drawing-side chamber thereof through thedirectional flow control valves for the bucket hydraulic cylinder, andthe hydraulic fluids from said second hydraulic pumps are supplied tothe rod drawing-side chamber thereof through said bucket-dumping inflowcontrol valve, and being joined with the hydraulic fluids from the firsthydraulic pumps, and with respect to the opening/closing hydrauliccylinder, the hydraulic fluids from said first hydraulic pumps aresupplied to said rod drawing-side chamber thereof through thedirectional flow control valves for the opening/closing hydrauliccylinder, while the remaining part of the fluids discharged from the rodpushing-side chambers of the boom and arm hydraulic cylinders and allfluids discharged from the rod pushing-side chambers of the bucket andopening/closing hydraulic cylinders are returned to said reservoir onlythrough the directional flow control valves for the boom, arm, bucketand opening/closing hydraulic cylinders.
 14. A hydraulic drive systemfor a construction machine comprising a travel body, a swing bodyswingably mounted onto said travel body, and a multi-articulated frontoperating mechanism made up of a boom rotatably coupled to said swingbody, an arm rotatably coupled to said boom, and a bucket rotatablycoupled to said arm, wherein said hydraulic drive system comprises: aboom hydraulic cylinder, an arm hydraulic cylinder, and a buckethydraulic cylinder for driving said boom, said arm, and said bucket,respectively; at least one hydraulic pump mounted on said swing body; acommon high-pressure line having one side connected to the delivery sideof said at least one hydraulic pump and the other side extended to theside of said front operating mechanism; a boom branch line branched fromsaid common high-pressure line and connected on the side opposite to thebranched side to a rod pushing-side chamber of said boom hydrauliccylinder; a boom inflow control valve disposed near a branch position atwhich said boom branch line is branched from said common high-pressureline, and controlling a flow of a hydraulic fluid supplied from saidcommon high-pressure line to the rod pushing-side chamber of said boomhydraulic cylinder; an arm branch line branched from said commonhigh-pressure line at a position downstream of the branch position ofsaid boom branch line and connected on the side opposite to the branchedside to a rod pushing-side chamber of said arm hydraulic cylinder; anarm inflow control valve disposed near a branch position at which saidarm branch line is branched from said common high-pressure line, andcontrolling a flow of a hydraulic fluid supplied from said commonhigh-pressure line to the rod pushing-side chamber of said arm hydrauliccylinder; a bucket branch line branched from said common high-pressureline at a position downstream of the branch position of said boom branchline and connected on the side opposite to the branched side to a rodpushing-side chamber of said bucket hydraulic cylinder; and a bucketinflow control valve disposed near the branch position at which saidbucket branch line is branched from said common high-pressure line, andcontrolling a flow of a hydraulic fluid supplied from said commonhigh-pressure line to the rod pushing-side chamber of said buckethydraulic cylinder; and wherein: said inflow control valves are alldisposed together in one control valve unit.
 15. A hydraulic drivesystem for a construction machine according to claim 14, wherein: saidone control valve unit is disposed on said boom.
 16. A hydraulic drivesystem for a construction machine comprising a travel body, a swing bodyswingably mounted onto said travel body, and a multi-articulated frontoperating mechanism made up of a boom rotatably coupled to said swingbody, an arm rotatably coupled to said boom, and a bucket rotatablycoupled to said arm, wherein said hydraulic drive system comprises: aboom hydraulic cylinder, an arm hydraulic cylinder, and a buckethydraulic cylinder for driving said boom, said arm, and said bucket,respectively; at least one hydraulic pump mounted on said swing body; acommon high-pressure line having one side connected to the delivery sideof said at least one hydraulic pump and the other side extended to theside of said front operating mechanism; a boom branch line branched fromsaid common high-pressure line and connected on the side opposite to thebranched side to a rod pushing-side chamber of said boom hydrauliccylinder; a boom inflow control valve disposed near a branch position atwhich said boom branch line is branched from said common high-pressureline, and controlling a flow of a hydraulic fluid supplied from saidcommon high-pressure line to the rod pushing-side chamber of said boomhydraulic cylinder; an arm branch line branched from said commonhigh-pressure line at a position downstream of the branch position ofsaid boom branch line and connected on the side opposite to the branchedside to a rod pushing-side chamber of said arm hydraulic cylinder; anarm inflow control valve disposed near a branch position at which saidarm branch line is branched from said common high-pressure line, andcontrolling a flow of a hydraulic fluid supplied from said commonhigh-pressure line to the rod pushing-side chamber of said arm hydrauliccylinder; a bucket branch line branched from said common high-pressureline at a position downstream of the branch position of said boom branchline and connected on the side opposite to the branched side to a rodpushing-side chamber of said bucket hydraulic cylinder; and a bucketinflow control valve disposed near the branch position at which saidbucket branch line is branched from said common high-pressure line, andcontrolling a flow of a hydraulic fluid supplied from said commonhigh-pressure line to the rod pushing-side chamber of said buckethydraulic cylinder; wherein said hydraulic drive system furthercomprises at least one of three sets comprising: a boom return fluidjoining line branched from said boom branch line at a position nearer tosaid boom hydraulic cylinder than said boom inflow control valve andconnected on the side opposite to the branched side to a hydraulicreservoir, and a boom outflow control valve disposed in said boom returnfluid joining line near a branch position at which said boom returnfluid joining line is branched from said boom branch line andcontrolling a flow of a hydraulic fluid drained from said boom hydrauliccylinder to said hydraulic reservoir; an arm return fluid joining linebranched from said arm branch line at a position nearer to said armhydraulic cylinder than said arm inflow control valve and connected onthe side opposite to the branched side to said hydraulic reservoir, andan arm outflow control valve disposed in said arm return fluid joiningline near a branch position at which said arm return fluid joining lineis branched from said arm branch line and controlling a flow of ahydraulic fluid drained from said arm hydraulic cylinder to saidhydraulic reservoir; and a bucket return fluid joining line branchedfrom said bucket branch line at a position nearer to said buckethydraulic cylinder than said bucket inflow control valve and connectedon the side opposite to the branched side to said hydraulic reservoir,and a bucket outflow control valve disposed in said bucket return fluidjoining line near a branch position at which said bucket return fluidjoining line is branched from said bucket branch line and controlling aflow of a hydraulic fluid drained from said bucket hydraulic cylinder tosaid hydraulic reservoir; and wherein: said inflow control valves andsaid outflow control valves are all disposed together in one controlvalve unit.
 17. A hydraulic drive system for a construction machineaccording to claim 16, wherein: said one control valve unit is disposedon said boom.
 18. A hydraulic drive system for a construction machinecomprising a travel body, a swing body swingably mounted onto saidtravel body, and a multi-articulated front operating mechanism made upof a boom rotatably coupled to said swing body, an arm rotatably coupledto said boom, and a bucket rotatably coupled to said arm to be openforward in a ground contact state, wherein said hydraulic drive systemcomprises: six first hydraulic pumps and two second hydraulic pumpsdriven by a plurality of prime movers; a boom hydraulic cylinder, an armhydraulic cylinder and a bucket hydraulic cylinder supplied withhydraulic fluids delivered from said first hydraulic pump and saidsecond hydraulic pump to drive said boom, said arm and said bucket,respectively, and an opening/closing hydraulic cylinder supplied withthe hydraulic fluids to open and close said bucket; a plurality of boomdirectional flow control valves, a plurality of arm directional flowcontrol valves, a plurality of bucket directional flow control valves,and a plurality of opening/closing directional flow control valves forcontrolling respective flows of the hydraulic fluids supplied from saidsix first hydraulic pumps to said boom hydraulic cylinder, said armhydraulic cylinder, said bucket hydraulic cylinder, and saidopening/closing hydraulic cylinder; a boom-raising inflow control valve,a bucket-crowding inflow control valve and a bucket-dumping inflowcontrol valve for controlling respective flows of the hydraulic fluidsdelivered from said two second hydraulic pumps and supplied to a rodpushing-side chamber of said boom hydraulic cylinder, a rod pushing-sidechamber of said bucket hydraulic cylinder, and a rod drawing-sidechamber of said bucket hydraulic cylinder without passing said pluralityof boom directional flow control valves and said plurality of bucketdirectional flow control valves; a bypass flow control valve forreturning the hydraulic fluids delivered from said two second hydraulicpumps to a reservoir; a boom recovery flow control valve and an armrecovery flow control valve for introducing the hydraulic fluids in therod pushing-side chambers of said boom hydraulic cylinder and said armhydraulic cylinder to rod drawing-side chambers thereof; and anopening/closing recovery flow control valve for introducing thehydraulic fluid in a rod drawing-side chamber of said opening/closinghydraulic cylinder to a rod pushing-side chamber thereof.
 19. Ahydraulic drive system for a construction machine according to claim 18,wherein: said inflow control valves are all disposed together in onecontrol valve unit.
 20. A hydraulic drive system for a constructionmachine according to claim 19, wherein: said one control valve unit isdisposed on said boom.
 21. A hydraulic drive system for a constructionmachine comprising a travel body, a swing body swingably mounted ontosaid travel body, and a multi-articulated front operating mechanismcoupled to said swing body in a vertically angularly movable manner andmade up of a boom, an arm and a bucket, wherein said hydraulic drivesystem comprises: a first hydraulic pump and a second hydraulic pumpdriven by prime movers; a plurality of hydraulic cylinders including aboom hydraulic cylinder, an arm hydraulic cylinder and a buckethydraulic cylinder supplied with hydraulic fluids delivered from saidfirst hydraulic pump and said second hydraulic pump to drive said boom,said arm, and said bucket, respectively; a plurality of directional flowcontrol valves for controlling respective flows of the hydraulic fluidsupplied from said first hydraulic pump to said plurality of hydrauliccylinders; at least one inflow control valve for controlling a flow ofthe hydraulic fluid delivered from said second hydraulic pump andsupplied to a rod pushing-side chamber of at least said boom hydrauliccylinder among said plurality of hydraulic cylinders without passingsaid directional flow control valves; a bypass flow control valve forreturning the hydraulic fluid delivered from said second hydraulic pump;and at least one recovery flow control valve for introducing thehydraulic fluid in the rod pushing-side chamber of at least said boomhydraulic cylinder among said plurality of hydraulic cylinders to a roddrawing-side chamber thereof; and wherein: said inflow control valvesare all disposed together in one control valve unit.
 22. A hydraulicdrive system for a construction machine according to claim 21, wherein:said one control valve unit is disposed on said boom.
 23. A hydraulicdrive system for a construction machine comprising a travel body, aswing body swingably mounted onto said travel body, and amulti-articulated front operating mechanism made up of a boom rotatablycoupled to said swing body, an arm rotatably coupled to said boom, and abucket rotatably coupled to said arm to be open forward in a groundcontact state, wherein said hydraulic drive system comprises: at leastone first hydraulic pump and at least one second hydraulic pump drivenby a plurality of prime movers; a plurality of hydraulic cylindersincluding a boom hydraulic cylinder, an arm hydraulic cylinder and abucket hydraulic cylinder supplied with hydraulic fluids delivered fromsaid first hydraulic pump and said second hydraulic pump to drive saidboom, said arm and said bucket, respectively, and an opening/closinghydraulic cylinder supplied with the hydraulic fluids to open and closesaid bucket; a plurality of directional flow control valves forcontrolling respective flows of the hydraulic fluid supplied from saidfirst hydraulic pump to said plurality of hydraulic cylinders; at leasttwo inflow control valve for controlling respective flows of thehydraulic fluid delivered from said second hydraulic pump and suppliedto rod pushing-side chambers of at least said boom hydraulic cylinderand said bucket hydraulic cylinder among said plurality of hydrauliccylinders without passing said directional flow control valves; a bypassflow control valve for returning the hydraulic fluid delivered from saidsecond hydraulic pump to a reservoir; and at least two recovery flowcontrol valve for introducing the hydraulic fluids in the rodpushing-side chambers of at least said boom hydraulic cylinder and saidarm hydraulic cylinder among said plurality of hydraulic cylinders torod drawing-side chambers thereof; and wherein: said inflow controlvalves are all disposed together in one control valve unit.
 24. Ahydraulic drive system for a construction machine according to claim 23,wherein: said one control valve unit is disposed on said boom.
 25. Ahydraulic drive system for a construction machine comprising a travelbody, a swing body swingably mounted onto said travel body, and amulti-articulated front operating mechanism made up of a boom rotatablycoupled to said swing body, an arm rotatably coupled to said boom, and abucket rotatably coupled to said arm to be open rearward in a groundcontact state, wherein said hydraulic drive system comprises: at leastone first hydraulic pump and at least one second hydraulic pump drivenby a plurality of prime movers; a plurality of hydraulic cylindersincluding a boom hydraulic cylinder, an arm hydraulic cylinder and abucket hydraulic cylinder supplied with hydraulic fluids delivered fromsaid first hydraulic pump and said second hydraulic pump to drive saidboom, said arm and said bucket, respectively; a plurality of directionalflow control valves for controlling respective flows of the hydraulicfluid supplied from said first hydraulic pump to said plurality ofhydraulic cylinders; a plurality of inflow control valve for controllingrespective flows of the hydraulic fluid delivered from said secondhydraulic pump and supplied to rod pushing-side chambers of said boomhydraulic cylinders, said arm hydraulic cylinder and said buckethydraulic cylinder without passing said directional flow control valves;a bypass flow control valve for returning the hydraulic fluid deliveredfrom said second hydraulic pump to a reservoir; and at least onerecovery flow control valve for introducing the hydraulic fluid in therod pushing-side chamber of at least said boom hydraulic cylinder amongsaid plurality of hydraulic cylinders to a rod drawing-side chamberthereof; and wherein: said inflow control valves are all disposedtogether in one control valve unit.
 26. A hydraulic drive system for aconstruction machine according to claim 25, wherein: said one controlvalve unit is disposed on said boom.